Add Rocky and Alma cells to the test matrices

We're replacing Centos 8 with Rocky and Alma support in 7.x. We should test the RHEL packages on these hosts, in FOSS and PE, to make sure they're as compatible as they claim.

I don't have strong opinions about where we add these. We weren't previously bothering to test on Centos 8 in most places, so it won't just be a replacement of those cells. Maybe just to smoke tests in both the Puppetserver and pe-pse pipelines? And/or as one of the compilers in the more complex pe-pse pipelines?

We'll need to make sure that our jobs know that these new platforms should use EL 8 packages.