UI notification/alert for projects on legacy composition

[n1ru4l]

Add a clear indicator when a project is still using the legacy federation composition workflow, along with guidance encouraging migration to the native federation composition approach.

This applies only to existing projects created before native federation composition became the default, as all new projects already use the native workflow.

Providing this visibility would help prevent confusion during project reviews or re-evaluations, where customers may encounter issues caused by older projects still relying on the legacy composition method.

[ben-m-klein]

I'll work on a PR for this notification soon.