Fix notification for new contributors

[nilsvu]

This automation hasn't ever really worked because of security restrictions by GitHub. But this change should fix it, see here: https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target

Proposed changes

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

[nilsvu]

Actually I'm just merging this because there's no way to test this anyway and it has no other impact whatsoever