PR metadata validation: Ensure "Complete" was added by a mentor not a trainee

Every now and then a trainee adds the "Complete" label themselves.

This can't be prevented by permissions on GitHub - labels are all or nothing.

But the [PR Metadata Validator](https://github.com/CodeYourFuture/trainee-tracker/blob/main/src/bin/pr-metadata-validator.rs) runs on every label change. We could in theory, when "Complete" is added, check if the user who took the action is (transitively) in [the "Mentors" team](https://github.com/orgs/CodeYourFuture/teams/mentors), and remove the label (with a comment) if they're not.