Cleanup & Fixes for .NET 5 (#309)

- [X] Fixes several bugs
- [X] Updates more ordinal issues
- [X] Cleans up the code a bit
- [X] Turns classes static that should have been
- [X] Changes TcpServer.Instances to a HashSet

Bumps release version

Projects/Server/Json/Converters/NullableStructSerializerFactory.cs

@@ -25,7 +25,9 @@ namespace System.Text.Json.Serialization
             }
 
             var structType = typeToConvert.GenericTypeArguments[0];
-            return !structType.IsPrimitive && structType.Namespace?.StartsWith(nameof(System)) != true && !structType.IsEnum;
+            return !structType.IsPrimitive &&
+                   structType.Namespace?.StartsWith(nameof(System), StringComparison.Ordinal) != true &&
+                   !structType.IsEnum;
         }
 
         public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options) =>