Add support for Enterprise Linux 9

Dear library maintainers,

the CMS GEM group is in the process of updating its online software to AlmaLinux 9, both for test stands and for the production system at P5. Among our dependencies, the AMC13 libraries are not yet officially built to support any Enterprise Linux 9 variant.

This MR thus aims at bringing support for all ABI-compatible Enterprise Linux 9 variants (AlmaLinux, RedHat,...) Following an approach similar to !13 (merged), only build errors are fixed and CI jobs added. No functional changes are expected outside of the centos7/8 to el7/8 rename in the RPM release names.

Please let me know if any additional information is needed.

Thank you!

Note: The RPM publishing jobs are falling since running in the context of my own fork.