Add conditional MapOpenApi for .NET 9.0 or newer

Introduced a preprocessor directive `#if NET9_0_OR_GREATER` to conditionally include the `app.MapOpenApi();` method call. This ensures that the `MapOpenApi` functionality is only executed when the application targets .NET 9.0 or later, maintaining compatibility with earlier .NET versions.

EnvelopeGenerator.API/Program.cs

@@ -290,7 +290,9 @@ try
 
     app.UseMiddleware<ExceptionHandlingMiddleware>();
 
+#if NET9_0_OR_GREATER
     app.MapOpenApi();
+#endif
 
     // Configure the HTTP request pipeline.
     if (app.Environment.IsDevelopment() || (app.IsDevOrDiP() && config.GetValue<bool>("UseSwagger")))