Add RHEL 8 queue(s)

[richlander]

RHEL 7 is now EOL (per our purposes). Context: https://www.redhat.com/en/technologies/linux-platforms/enterprise-linux/rhel-7-end-of-maintenance

We need to move all RHEL testing for all supported .NET versions to RHEL 8 but we don't have a RHEL 8 queue.

Here is a PR I created to move the test legs. I didn't realize the lack of a RHEL 8 queue when I started. Ooops.

dotnet/runtime#105063

[dougbu]

@richlander you're now testing w/ CentOS Docker containers. is a RedHat 8 test queue still useful❓

/cc @ilyas1974

[richlander]

Great question. I would like to skip this if we could. I propose we wait until we have a pressing need.