CORS with endpoint routing and RequireCors does not support automatic preflight requests

This was first reported in #18665
See [Enable Cors with endpoint routing](https://docs.microsoft.com/en-us/aspnet/core/security/cors?view=aspnetcore-3.1#enable-cors-with-endpoint-routing) which explains it and has deployed code to reproduce the behavior. Given this is documented you may want to close this issue by design and I'll update the docs to say ***By Design**.

cc @pranavkm  @apacurariu 