Switch to the images provide by the Linux support group and use the alma9 image for el9

It seems Rocky 9 will not be supported, at least on a short timescale by the Linux Support group. We initially switch to Alma 9, which should give compatible results. If that's the case, then we can consider keeping it for el9.

Here we also switch to using the images provided by the Linux support group, which can be found in in https://gitlab.cern.ch/linuxsupport. We also drop the support for the slc6 image, which is no longer used by any project.