Define RHEL8 helix machines

[sbomer]

With the plan in dotnet/runtime#83428 (implemented in dotnet/runtime#84148), .NET 8 minimum glibc version is glibc 2.23. This means RHEL7 is no longer supported.

Some teams are using RHEL7 helix images for testing (for example, see failing aspnetcore tests using Redhat.7.Amd64.Open in https://dev.azure.com/dnceng-public/public/_build/results?buildId=237929&view=results). We need new RHEL8 images to replace these.

cc @captainsafia

[sbomer]

@jkoritzinsky and @richlander suggested using an existing helix queue with a container image instead, so we may not need to define a new helix queue - so I'll close this and work on adding new image definitions instead.

[captainsafia]

cc: @JamesNK who is currently on blops. Something to keep in mind in case the new image definitions happen during your rotation...

[ManickaP]

Should we consider upgrading RH 7 helix machines to either RH 8 or 9? RH 7 is fairly old release that is out full support (in maintenance): https://access.redhat.com/support/policy/updates/errata#Life_Cycle_Dates

[richlander]

Good idea, but I'd suggest not doing that, for two reasons:

• .NET 6 needs to guarantee RHEL 7 support through its life.
• We plan to use Alma 8+ container images to validate RHEL 8+ quality going forward (at least for .NET 8+).

Once .NET 6 is EOL, we can decommission RHEL 7 machines.