Drop RHEL7 and add RHEL9 support

[piste-jp]

May be it is a time to move forward on the master branch.

• Drop RHEL7 support
• Drop ArchLinux support from official
  • Rolling model is nasty for the project, we cannot describe which point in the OS side can be compilable and usable...
• Add RHEL9 support (implicitly rocky linux 8)
• Refresh other old Linux distros to the latest version
  • Ubuntu 24.04 LTS
  • Debian 11 LTS
• May be some Linux distro to consider to add
  • Non LTS Ubuntu (is it too aggressive??)
  • Non LTF Debian (is it too aggressive??)
  • Fedora (is it too aggressive??)

[jgreen24]

For the stable branch at least, the plan is to drop RHEL 7 support by July 1st, 2024 (EOS is June 30th). I think it would be a good time for the master branch as well.

RHEL 9.0 support for stable branch also is coming soon, so I don't think it will be a problem to test for master at the same time.

[piste-jp]

Hi @jgreen24 , Thank you for your information about the plan on the v2.4-stable branch.

I would like to clarify what I'm thinking now.

• Define first target and second target in this project
  • Now they are implicitly described into the README, but I would like to have a clear statement
  • First target will be RHEL9 and second target will be RHEL8
  • Warning check (build and warning check) would be done in the first target
  • Build check (only build) would be done in the second target
  • RHEL check should be done in Rocky Linux, a clone of RHEL
• Other Linux distros are defined as 'low priority targets'
  • Make a build check but do not fix breakages actively
• Modify configure.ac for using appropriate version of automake/autoconf on RHEL9/8
• Remove all warnings on RHEL9's default gcc compiler, ld linker and libtool
• Drop the description of ppc64le from the target list
• Prepare new build checker environment
  • Top priority
    • Rocky Linux 9 (clone of RHEL9)
    • Rocky Linux 8 (clone of RHEL8)
  • Second priority
    • Ubuntu 24.04 LTS
    • Debian 11 LTS
  • Low priority
    • Non LTS Ubuntu
    • Non LTF Debian
    • Fedora
    • Arch Linux