Add OpenAPI service for .NET 9.0 or newer

Introduced a conditional compilation directive (`#if NET9_0_OR_GREATER`) to register the `AddOpenApi` service only when the application is targeting .NET 9.0 or later. This ensures compatibility and avoids unnecessary service registration for earlier .NET versions.

EnvelopeGenerator.API/Program.cs

@@ -111,7 +111,9 @@ try
 
         options.DocumentFilter<EnvelopeGenerator.API.Documentation.AuthProxyDocumentFilter>();
     });
+#if NET9_0_OR_GREATER
     builder.Services.AddOpenApi();
+#endif
 
     //Add EF Core dbcontext
     var useDbMigration = Environment.GetEnvironmentVariable("MIGRATION_TEST_MODE") == true.ToString() || config.GetValue<bool>("UseDbMigration");