The original event used, 'pull_request', runs in the context of the
pull request. In the case of pull requests created from forks this means
that a read-only GitHub token is used for API calls, and thus the
call used to assign the milestone fails.
The fix is using the 'pull_request_target' event, which runs in the
context of the base branch, and thus with a read-write token. See:
https://docs.github.com/en/actions/reference/events-that-trigger-workflows#pull_request_target
Also, added a warning message when no milestone without a release
branch is found and thus the newest existing milestone is assigned
(this is a fallback that should normally not happen).
d48775f440