run CI weekly against supported release branches

[skriss]

Currently, there is no scheduled CI against our three supported release branches.This can lead to them rotting and not being healthy when it's time to do a patch release.

We should run some form of CI against them regularly. A brief investigation reveals that it may not be possible to set up a scheduled run against a non-default branch. I'm wondering if instead, we could add a scheduled run on main to open a no-op PR against each of them weekly, to exercise the prbuild.yaml workflow. Open to other options that exercise the workflows in the branches themselves as well.