CI updates (master branch)

[mhilbrunner]

Happy 2025! Time for some spring cleaning in the new year:

• Update from Ubuntu 22.04 to 24.04 (and replace all use of ubuntu-latest with 24.04 for consistency)
  • Update workflows to use Python venvs, as newer Ubuntu (and Debian) versions don't want us trashing global packages with Python packages (see here)
    • Using virtualenvs also means we now need to exclude directories starting with . from Sphinx builds, to support .venv (this would have been a good idea anyway, for .git, .vscode and so on)
• Fix permission issues (by adding write permissions to repo contents and/or PRs where those are now needed, see here, originally added around 2021, now mandatory), this should hopefully fix the issues with Add GitHub Action to automatically create cherrypick PRs #10361)
• (Hopefully) improve CI build times by utilizing up to 4 CPUs (which according to docs seems now save to assume for Github action runners)
• Change all references to github.ref to github.ref_name, which should be equivalent to the old behaviour (see here)
• Set timeouts (by default, Github waits 6 hours before killing a hung job, whereas all our stuff should be killed with fire way earlier, most take a few minutes a most, with building the offline docs being the longest at 1-2 hours, not 6)
• Update some comments that have grown old by now

[mhilbrunner]

Let's see if anything breaks, all tests so far seem good 👀 Merging, as I want to do further work on GH actions and this needs to go in first... thanks for having a look, tetra!