improve support for rockylinux 8 and add a few simplifications

[michaelmayer2]

After realizing that r-lib/pkgdepends#397 was the wrong repository, trying my luck here.

This PR will bring better support for RockyLinux 8 - main story here is that RockyLinux 8 does not use crb but powertools as Code Ready Builder repo.

Where appropriate I was also able to relax the system requirements as far as epel-release and crb/powertools is concerned.

Also contains a fix for the atkmm-dev issue with Alpine Edge in 6eb53f4 which will make GHA happy again

[glin]

So.. because of the awkwardness with the CentOS 8 EOL, we switched CentOS 8 to Rocky 8 but left the distribution as centos to not break any existing users, as well as Package Manager and its users of the centos8 binary packages. That may be under-documented here as it's just a footnote in the readme I think, https://github.com/rstudio/r-system-requirements?tab=readme-ov-file#user-content-fn-1-a3305ad47d408ac599c56c33f6ed79e9

Would pkgdepends be fine with using centos-8 as an alias for Rocky Linux 8 (+related distros like Alma 8)?

If it's easier to just have specific rockylinux8 rules here, then I think that'd still be okay, since they'd be identical to all the existing centos8 rules. So it wouldn't be much more work to maintain.

[michaelmayer2]

The latest state of the PR now has fully functional rockylinux8 integration - probably easier to remove centos8 at some point when rockylinux8 is still a thing.

If nothing else comes out of this PR, you probably should rip out the changes to atk.json. This will make GHA work again.

[glin]

This looks good. Thanks!

And a heads up to @gaborcsardi that there is now explicit rockylinux-8 support here, in addition to the previous centos-8 alias