[chore][CI] Reducing qemu and Ubuntu 20.04 usage

[pjanotti]

Continuing the work of #6052 and also reducing Ubuntu 20.04 usage

• Removing mentions of upstream issue 16450 that doesn't apply anymore. This helps isolate and understand some workflows that rely on Ubuntu 20.04
• Removing more qemu usage
• Separating out the cases (Amazon Linux 2, OpenSUSE 12 (aka leap 42), and "tar" CentOS 7 that do require Ubuntu runner 20.04 (cgroup v1 requirements of the respective images)
• Notice that OpenSUSE 12 and CentOS 7 already reached EOL and I will remove them in a follow-up. Not doing it here to keep the history clean and not mix up the deletion of container files with GH workflow changes

Note: the Puppet rake job failure is a known issue and expected