Align MigrateFromNamedSingleTopic obsolete version range (#1209)

* Align MigrateFromNamedSingleTopic obsolete version range with the other obsoletes since that is an oversight from a bumping PR.

Introduces also constants for the version range with comments to better indicate the intent.

* Use constants in MigrationTopology class also

---------

Co-authored-by: Daniel Marbach <[email protected]>
Co-authored-by: Brandon Ording <[email protected]>

Author: danielmarbach

src/Tests/ApprovalFiles/APIApprovals.Approve.approved.txt

@@ -121,7 +121,7 @@ namespace NServiceBus
         public static NServiceBus.Transport.AzureServiceBus.MigrationTopology MigrateFromNamedSingleTopic(string topicName) { }
         [System.Obsolete(@"The migration topology is intended to be used during a transitional period, facilitating the migration from the single-topic topology to the topic-per-event topology. The migration topology will eventually be phased out over subsequent releases. Should you face challenges during migration, please reach out to |https://github.com/Particular/NServiceBus.Transport.AzureServiceBus/issues/1170|. Will be treated as an error from version 7.0.0. Will be removed in version 8.0.0.", false)]
         public static NServiceBus.Transport.AzureServiceBus.MigrationTopology MigrateFromSingleDefaultTopic() { }
-        [System.Obsolete(@"The migration topology is intended to be used during a transitional period, facilitating the migration from the single-topic topology to the topic-per-event topology. The migration topology will eventually be phased out over subsequent releases. Should you face challenges during migration, please reach out to |https://github.com/Particular/NServiceBus.Transport.AzureServiceBus/issues/1170|. Will be treated as an error from version 6.0.0. Will be removed in version 7.0.0.", false)]
+        [System.Obsolete(@"The migration topology is intended to be used during a transitional period, facilitating the migration from the single-topic topology to the topic-per-event topology. The migration topology will eventually be phased out over subsequent releases. Should you face challenges during migration, please reach out to |https://github.com/Particular/NServiceBus.Transport.AzureServiceBus/issues/1170|. Will be treated as an error from version 7.0.0. Will be removed in version 8.0.0.", false)]
         public static NServiceBus.Transport.AzureServiceBus.MigrationTopology MigrateFromTopicHierarchy(string topicToPublishTo, string topicToSubscribeOn) { }
         [System.Obsolete("Use `TopicTopology.MigrateFromNamedSingleTopic(string topicName)` instead. The me" +
             "mber currently throws a NotImplementedException. Will be removed in version 6.0." +

src/Transport/EventRouting/MigrationTopology.cs

@@ -6,7 +6,7 @@ namespace NServiceBus.Transport.AzureServiceBus;
 /// <summary>
 /// Topology that allows mixing of single-topic and topic-per-event approaches in order to allow gradual migration to the topic-per-event topology.
 /// </summary>
-[ObsoleteEx(Message = ObsoleteMessage, TreatAsErrorFromVersion = "7", RemoveInVersion = "8")]
+[ObsoleteEx(Message = ObsoleteMessage, TreatAsErrorFromVersion = TreatAsErrorFromVersion, RemoveInVersion = RemoveInVersion)]
 public sealed class MigrationTopology : TopicTopology
 {
     internal MigrationTopology(MigrationTopologyOptions options) : base(options,
@@ -187,6 +187,17 @@ protected override string GetPublishDestinationCore(string eventTypeFullName)
 
     internal override SubscriptionManager CreateSubscriptionManager(SubscriptionManagerCreationOptions creationOptions) => new MigrationTopologySubscriptionManager(creationOptions, Options);
 
+    /*
+     * The idea behind obsoleting the migration topology is to make it clear in the user's code base that eventually the migration topology will be removed.
+     * The intent was to give users at least two major versions that are aligned with .NET LTS versions time, which should be roughly 2 years to migrate away
+     * from the migration topology to the new topic-per-event topology. Should an out-of-band major release be required, consider bumping the `TreatAsErrorFromVersion`
+     * and `RemoveInVersion` values to make sure more or least two years have passed after the release v5.0.0 of the transport. It might also be worthwhile
+     * checking the linked issue in the obsolete message to see if there are any signals that warrant moving this even further.
+     */
+    internal const string TreatAsErrorFromVersion = "7";
+
+    internal const string RemoveInVersion = "8";
+
     internal const string ObsoleteMessage =
         "The migration topology is intended to be used during a transitional period, facilitating the migration from the single-topic topology to the topic-per-event topology. The migration topology will eventually be phased out over subsequent releases. Should you face challenges during migration, please reach out to |https://github.com/Particular/NServiceBus.Transport.AzureServiceBus/issues/1170|.";
 

src/Transport/EventRouting/MigrationTopologyOptions.cs

@@ -6,7 +6,7 @@ namespace NServiceBus.Transport.AzureServiceBus;
 /// <summary>
 /// Serializable object that defines the migration topology
 /// </summary>
-[ObsoleteEx(Message = MigrationTopology.ObsoleteMessage, TreatAsErrorFromVersion = "7", RemoveInVersion = "8")]
+[ObsoleteEx(Message = MigrationTopology.ObsoleteMessage, TreatAsErrorFromVersion = MigrationTopology.TreatAsErrorFromVersion, RemoveInVersion = MigrationTopology.RemoveInVersion)]
 public sealed class MigrationTopologyOptions : TopologyOptions
 {
     /// <summary>

src/Transport/EventRouting/MigrationTopologyOptionsValidator.cs

@@ -5,6 +5,6 @@ namespace NServiceBus.Transport.AzureServiceBus;
 /// <summary>
 /// Validates the <see cref="MigrationTopologyOptions"/>.
 /// </summary>
-[ObsoleteEx(Message = MigrationTopology.ObsoleteMessage, TreatAsErrorFromVersion = "7", RemoveInVersion = "8")]
+[ObsoleteEx(Message = MigrationTopology.ObsoleteMessage, TreatAsErrorFromVersion = MigrationTopology.TreatAsErrorFromVersion, RemoveInVersion = MigrationTopology.RemoveInVersion)]
 [OptionsValidator]
 public partial class MigrationTopologyOptionsValidator : IValidateOptions<MigrationTopologyOptions>;

src/Transport/EventRouting/TopicTopology.cs

@@ -46,14 +46,14 @@ public static TopicTopology FromOptions(TopologyOptions options) =>
         /// <summary>
         /// Returns a migration topology using a single topic named <c>bundle-1</c> for <see cref="MigrationTopology.TopicToPublishTo"/> and <see cref="MigrationTopology.TopicToSubscribeOn"/>
         /// </summary>
-        [ObsoleteEx(Message = MigrationTopology.ObsoleteMessage, TreatAsErrorFromVersion = "7", RemoveInVersion = "8")]
+        [ObsoleteEx(Message = MigrationTopology.ObsoleteMessage, TreatAsErrorFromVersion = MigrationTopology.TreatAsErrorFromVersion, RemoveInVersion = MigrationTopology.RemoveInVersion)]
         public static MigrationTopology MigrateFromSingleDefaultTopic() => MigrateFromNamedSingleTopic("bundle-1");
 
         /// <summary>
         /// Returns a migration topology using a single topic with the <paramref name="topicName"/> for <see cref="MigrationTopology.TopicToPublishTo"/> and <see cref="MigrationTopology.TopicToSubscribeOn"/>
         /// </summary>
         /// <param name="topicName">The topic name.</param>
-        [ObsoleteEx(Message = MigrationTopology.ObsoleteMessage, TreatAsErrorFromVersion = "7", RemoveInVersion = "8")]
+        [ObsoleteEx(Message = MigrationTopology.ObsoleteMessage, TreatAsErrorFromVersion = MigrationTopology.TreatAsErrorFromVersion, RemoveInVersion = MigrationTopology.RemoveInVersion)]
         public static MigrationTopology MigrateFromNamedSingleTopic(string topicName) => new(new MigrationTopologyOptions
         {
             TopicToPublishTo = topicName,
@@ -67,7 +67,7 @@ public static TopicTopology FromOptions(TopologyOptions options) =>
         /// <param name="topicToSubscribeOn">The topic name to subscribe to.</param>
         /// <returns></returns>
         /// <exception cref="ArgumentException">Thrown when <paramref name="topicToPublishTo"/> is equal to <paramref name="topicToSubscribeOn"/>.</exception>
-        [ObsoleteEx(Message = MigrationTopology.ObsoleteMessage, TreatAsErrorFromVersion = "6", RemoveInVersion = "7")]
+        [ObsoleteEx(Message = MigrationTopology.ObsoleteMessage, TreatAsErrorFromVersion = MigrationTopology.TreatAsErrorFromVersion, RemoveInVersion = MigrationTopology.RemoveInVersion)]
         public static MigrationTopology MigrateFromTopicHierarchy(string topicToPublishTo, string topicToSubscribeOn)
         {
             var hierarchy = new MigrationTopology(new MigrationTopologyOptions