CI: move upstream-compat checks to other repo
Right now, we have a bunch of upstream-compatibility checks in pmaports.git's CI. They fail if there was an upgrade in Alpine, for example an upgrade of grub-efi.
Since the upstream-compat checks only run on master and not during merge requests, it's confusing if a patch passes CI in the MR, then gets merged, and then CI fails for a completely different reason that has nothing to do with the patch.
So it would be nice if we moved the checks to elsewhere. Afaik it is possible now to trigger CI jobs in gitlab on a given time interval, such as daily. Therefore a simple solution would be moving all upstream-compat checks to a new upstream-compat repository and running it every day (or hour?).
We also need notifications, either via e-mail or via IRC/matrix.