Add venv cache versioning to CI

[janbrasna]

To avoid CI failures during template upgrades (or just Python bumps if consumers do so) the venv created using older Python version must not be restored from cache.

Adding a Python version key seems easier than trying to document the fact consumers need to remember manually going into their Actions looking into the right Caches and deleting all of them (which also needs certain permissions for the repo, so can't be e.g. ensured from just a PR doing the upgrade etc.)

Doing some extra/fragile shenanigans to be more lax and only key to major.minor could be viable, but this achieves precise version for the sake of simplicity, given older Python versions don't really get frequent updates (compared to the useful cache lifetime):

This also updates actions versions to prepare for upcoming runner deprecations.