GHA: complain if PR adds commits from people not yet listed in ./AUTHORS

[Al2Klimov]

not to have to update ./AUTHORS or .mailmap after merging.

This substitutes ProBot functionality.

[Al2Klimov]

Not critically necessary, but should prevent stuff like #9697 (comment).

[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Lol cat.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email email@example.com
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails

[julianbrost]

Looks fine in the failing case, please remove the second commit so that we can also see it working if everything is fine.