Add GitHub Actions workflow for labeling deployment PRs

[ddabble]

Proposed changes

New features

• Added GitHub Actions workflow for automatically labeling deployment PRs (e72a6bc53ba5800790b140c667bba58ff175806c)

Improvements

• Made use of GitHub Actions config variables (e9b2d108df3684120d0d390420b0ffd84d01265d)
  • See https://github.blog/changelog/2023-01-10-github-actions-support-for-configuration-variables-in-workflows/

Other changes

• Updated the CodeQL config to match the default one generated by GitHub (see the commit message for more details) (8e2a608dcfc8b882fc5fff7cf5ba03f698990472)
  • This mainly changed it to now be scheduled to run just once a week (every Monday at at 06:45 UTC), which seems sufficient

Areas to review closely

None in particular, as the added workflow has been tested to work in a (deleted) test repo.

Checklist

(If any of the points are not relevant, mark them as checked)

• [x] Created tests that fail without the changes, if relevant/possible
• [x] Made sure that your code conforms to the code style guides and that any common code smells have been addressed
  • (It's not intended that you read through this whole document, but that you get yourself an overview over its contents, and that you keep it in mind while taking a second look at your code before opening a pull request)
• [x] Added sufficient documentation - e.g. as docstrings or in the README, if suitable
• [x] Added your changes to the "Unreleased" section of the changelog - mainly the changes that are of particular interest to users and/or developers, if any
• [x] Added a "Deployment notes" section above, if anything out of the ordinary should be done when deploying these changes to the server

[ddabble]

Merging without explicit approval from another member, as the Dev committee agreed to merge these changes and assume they're relatively bug-free, simply to get things done quicker.