Mark some deprecated properties as obsolete in Schema (#2359)

These properties will be removed in a future major release.
These were actually marked as obsolete in the past, but were
temporarily removed.  This just re-adds the annotations.

Author: rayokota

src/Confluent.SchemaRegistry/Rest/DataContracts/RegisteredSchema.cs

@@ -41,19 +41,19 @@ public class RegisteredSchema : Schema, IComparable<RegisteredSchema>, IEquatabl
         ///     The subject the schema is registered against.
         /// </summary>
         [DataMember(Name = "subject")]
-        public override string Subject { get; set; }
+        public new string Subject { get; set; }
 
         /// <summary>
         ///     The schema version.
         /// </summary>
         [DataMember(Name = "version")]
-        public override int Version { get; set; }
+        public new int Version { get; set; }
 
         /// <summary>
         ///     Unique identifier of the schema.
         /// </summary>
         [DataMember(Name = "id")]
-        public override int Id { get; set; }
+        public new int Id { get; set; }
 
         /// <summary>
         ///     The unregistered schema corresponding to this schema.

src/Confluent.SchemaRegistry/Rest/DataContracts/Schema.cs

@@ -30,19 +30,22 @@ public class Schema : IComparable<Schema>, IEquatable<Schema>
         #region API backwards-compatibility hack
 
         /// <summary>
-        ///     The subject the schema is registered against.
+        ///     DEPRECATED. The subject the schema is registered against.
         /// </summary>
-        public virtual string Subject { get; set; }
+        [Obsolete("Included to maintain API backwards compatibility only. Use RegisteredSchema instead. This property will be removed in a future version of the library.")]
+        public string Subject { get; set; }
 
         /// <summary>
-        ///     The schema version.
+        ///     DEPRECATED. The schema version.
         /// </summary>
-        public virtual int Version { get; set; }
+        [Obsolete("Included to maintain API backwards compatibility only. Use RegisteredSchema instead. This property will be removed in a future version of the library.")]
+        public int Version { get; set; }
 
         /// <summary>
-        ///     Unique identifier of the schema.
+        ///     DEPRECATED. Unique identifier of the schema.
         /// </summary>
-        public virtual int Id { get; set; }
+        [Obsolete("Included to maintain API backwards compatibility only. Use RegisteredSchema instead. This property will be removed in a future version of the library.")]
+        public int Id { get; set; }
 
         /// <summary>
         ///     DEPRECATED. Initializes a new instance of the Schema class.