Support RHEL and CentOS 8

[wyardley]

Pull Request (PR) description

Add metadata support for RHEL / CentOS 8

Note: CentOS 8 has been EOL for a while, and RHEL 8 is EOL at the end of May, 2024. Thus, official support may be dropped again soon

This Pull Request (PR) fixes the following issues

Fixes #942
Closes #900
Fixes #816

[wyardley]

Yeah, looks like EPEL doesn't have it

We could maybe enable / install centos-release-rabbitmq-38, but I'm running into some erlang dep related issues even then

[root@centos8-64-1 /]# yum install rabbitmq-server --allowerasing
Last metadata expiration check: 0:00:22 ago on Mon 20 May 2024 12:28:53 AM UTC.
Error: 
 Problem: package erlang-common_test-22.0.7-1.el8.aarch64 from epel requires erlang-erts(aarch-64) = 22.0.7-1.el8, but none of the providers can be installed
  - package erlang-lager-3.8.0-2.el8.noarch from centos-rabbitmq-38 requires erlang-common_test, but none of the providers can be installed
  - cannot install both erlang-erts-22.0.7-1.el8.aarch64 from epel and erlang-erts-22.3.4.1-1.el8.aarch64 from centos-rabbitmq-38
  - package erlang-common_test-22.3.4.1-1.el8.aarch64 from centos-rabbitmq-38 requires erlang-observer(aarch-64) = 22.3.4.1-1.el8, but none of the providers can be installed
  - package erlang-sd_notify-1.0-14.1.el8.aarch64 from centos-rabbitmq-38 requires erlang(erl_nif_version) = 2.15, but none of the providers can be installed
  - package erlang-cuttlefish-2.0.11-10.el8.noarch from centos-rabbitmq-38 requires erlang-lager, but none of the providers can be installed
  - package erlang-observer-22.3.4.1-1.el8.aarch64 from centos-rabbitmq-38 requires erlang-wx(aarch-64) = 22.3.4.1-1.el8, but none of the providers can be installed
  - package rabbitmq-server-3.8.3-1.el8.aarch64 from centos-rabbitmq-38 requires erlang-sd_notify(aarch-64), but none of the providers can be installed
  - package rabbitmq-server-3.8.3-1.el8.aarch64 from centos-rabbitmq-38 requires erlang-cuttlefish, but none of the providers can be installed
  - package erlang-wx-22.3.4.1-1.el8.aarch64 from centos-rabbitmq-38 requires libwx_gtk3u_core-3.0.so.0(WXU_3.0.5)(64bit), but none of the providers can be installed
  - conflicting requests
  - nothing provides libSDL2-2.0.so.0()(64bit) needed by wxGTK3-3.0.5.1-1.el8.aarch64 from centos-rabbitmq-38
(try to add '--skip-broken' to skip uninstallable packages or '--nobest' to use not only best candidate packages)

At the least, the integration tests work for CentOS 8 so if someone wants to try adding support, it should be somewhat more feasible now, but I think the core issues remain until we change the default to repos_ensure.

I also don't know how we can unit test RHEL 9 since there's no corresponding CentOS 9 with the way things work these days.