[OpenAPI] Only enable XML source gen for C# (dotnet#62069)

Only enable the OpenAPI XML source generator for C#.

See dotnet#61069 (comment).

Author: martincostello

src/OpenApi/gen/XmlCommentGenerator.cs

@@ -7,7 +7,7 @@
 
 namespace Microsoft.AspNetCore.OpenApi.SourceGenerators;
 
-[Generator]
+[Generator(LanguageNames.CSharp)]
 public sealed partial class XmlCommentGenerator : IIncrementalGenerator
 {
     public void Initialize(IncrementalGeneratorInitializationContext context)