Add (daily/weekly) CI for supported release branches

description

A recurring issue we have is that there are some parts of CI that are not pinned, and are subject to change.

This can lead to bit-rot on release branches. Mostly these are just CI issues, but sometimes flag incompatibilities that should be addressed.

Currently this effects stable maintainers and backporters around the time of releases, who then have to figure what is breaking/broke to land their PRs

It would be very helpful if we were to add some CI that tests these branches regularly, and potentially alerts maintainers one way or another as issues arise.