Per https://www.mediawiki.org/wiki/GitLab_consultation we move repository hosting to GitLab.
Thus creating a dedicated task for repos currently canonically hosted on GitHub.

This task will need subtasks to identify and tackle repos on GitHub. Some GitHub repos are listed here; very likely incomplete because people tend to host stuff wherever they feel like. This makes any contribution guidelines confusing and harder to read due to if/else ("The maintainers of each software project are pretty free to choose the infrastructure they prefer. ... Examples are Wikimedia Git/Gerrit, GitHub, or Sourceforge").

There have been recurring discussions (due to lack of guidance and probably lack of authority) when repos are supposed to be hosted in Gerrit vs Github vs Differential (the latter is covered in T191182). For some examples, see comments in numerous tasks.

See also https://www.mediawiki.org/wiki/GitLab_consultation for arguments why (not) to use third-party hosting. Additional arguments seem to be receiving drive-by contributions (though I'm not aware of data proving that assumption); that some of our GitHub repos are reusable outside of WM (but I'm not aware of data if that really happens); that there is a bigger discoverability on GitHub (which might be invalid due to mirroring).