* ci: update to checkout v6
v4 and older causes warnings about deprecated node.js
* ci: update to actions cache v5
the older versions cause deprecation warings for node.js 20
Forks that would like to contribute via PRs from feature branches
needlessly run CI on those branches on top of the PRs.
This is a waste of resources.
Co-authored-by: friendlyanon <friendlyanon@users.noreply.github.com>