ci: Check PRs for semantic labels

[flying-sheep]

… and adapts towncrier fragments to match the labels

Also:

• cleans up CI (remove useless options, update actions)
• adds dependabot for automatic GH actions updates

Does not:

• check if PR title matches fragment (TODO? how to deal with breaking vs “!”)?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.91%. Comparing base (cfab469) to head (a99b21e).
⚠️ Report is 59 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3772   +/-   ##
=======================================
  Coverage   75.91%   75.91%           
=======================================
  Files         117      117           
  Lines       12569    12569           
=======================================
  Hits         9542     9542           
  Misses       3027     3027           

[flying-sheep]

Ah OK, we’re not using the publish job for CI because of stuff like

This branch had an error being deployed

got it. went back to duplicating the “build” job between the CI and publish workflow.