Clean up RedHat 7 Helix queue use in our repos

[dougbu]

Though RedHat 7 is EOL, dotnet/arcade and dotnet/aspnetcore continue to test on our redhat.7.amd64 queues in at least some branches. dotnet/runtime was a big user of these queues but has fortunately already moved to test with Docker containers for more current OS versions.

Release Note Category

• Feature changes/additions
• Bug fixes
• Internal Infrastructure Improvements

Release Note Description

Removed use of the following Helix queues: redhat.7.amd64, redhat.7.amd64.open, redhat.7.amd64.open.rt, redhat.7.amd64,open.svc, redhat.7.amd64.rt, and redhat.7.amd64.svc in the dotnet/arcade and dotnet/aspnetcore repositories due to the EOL status of the operating system.