Raise an error if Protobuf deprecated format is specified (#2479)

* Raise an error if Protobuf deprecated format is specified

* Minor cleanup

Author: rayokota

src/Confluent.SchemaRegistry.Serdes.Protobuf/ProtobufDeserializer.cs

@@ -47,8 +47,6 @@ namespace Confluent.SchemaRegistry.Serdes
     /// </remarks>
     public class ProtobufDeserializer<T> : AsyncDeserializer<T, FileDescriptorSet> where T : class, IMessage<T>, new()
     {
-        private bool useDeprecatedFormat;
-        
         private MessageParser<T> parser;
 
         /// <summary>
@@ -82,9 +80,9 @@ public ProtobufDeserializer(ISchemaRegistryClient schemaRegistryClient, Protobuf
             }
 
             ProtobufDeserializerConfig protobufConfig = new ProtobufDeserializerConfig(config);
-            if (protobufConfig.UseDeprecatedFormat != null)
+            if (protobufConfig.UseDeprecatedFormat != null && protobufConfig.UseDeprecatedFormat.Value)
             {
-                this.useDeprecatedFormat = protobufConfig.UseDeprecatedFormat.Value;
+                throw new NotSupportedException("ProtobufDeserializer: UseDeprecatedFormat is no longer supported");
             }
 
             if (config.UseLatestVersion != null) { this.useLatestVersion = config.UseLatestVersion.Value; }

src/Confluent.SchemaRegistry.Serdes.Protobuf/ProtobufSerializer.cs

@@ -53,7 +53,6 @@ namespace Confluent.SchemaRegistry.Serdes
     public class ProtobufSerializer<T> : AsyncSerializer<T, FileDescriptorSet>  where T : IMessage<T>, new()
     {
         private bool skipKnownTypes = true;
-        private bool useDeprecatedFormat;
         private ReferenceSubjectNameStrategyDelegate referenceSubjectNameStrategy;
 
         /// <remarks>
@@ -91,7 +90,10 @@ public ProtobufSerializer(ISchemaRegistryClient schemaRegistryClient, ProtobufSe
             if (config.UseLatestVersion != null) { this.useLatestVersion = config.UseLatestVersion.Value; }
             if (config.UseLatestWithMetadata != null) { this.useLatestWithMetadata = config.UseLatestWithMetadata; }
             if (config.SkipKnownTypes != null) { this.skipKnownTypes = config.SkipKnownTypes.Value; }
-            if (config.UseDeprecatedFormat != null) { this.useDeprecatedFormat = config.UseDeprecatedFormat.Value; }
+            if (config.UseDeprecatedFormat != null && config.UseDeprecatedFormat.Value)
+            {
+                throw new NotSupportedException("ProtobufSerializer: UseDeprecatedFormat is no longer supported");
+            }
             if (config.SubjectNameStrategy != null) { this.subjectNameStrategy = config.SubjectNameStrategy.Value.ToDelegate(); }
             if (config.SchemaIdStrategy != null) { this.schemaIdEncoder = config.SchemaIdStrategy.Value.ToEncoder(); }
             this.referenceSubjectNameStrategy = config.ReferenceSubjectNameStrategy == null