Add {macos,ubuntu,windows}-penulimate label

[jendrikseipp]

For people like me who'd like to always test the latest two OS versions, it would be great to have {macos,ubuntu,windows}-penulimate labels in addition to {macos,ubuntu,windows}-latest.

[mikemahoney218]

+1

In addition, I think it's a semi-common use-case to test on older OS versions, to ensure maximum compatibility; I'd love if there were a floating tag oldest that always pointed at the oldest-supported version of a given OS.

[mikhailkoliada]

Hello!

We do not currently have plans to introduce more labels I am afraid, so the only way to check what is penultimate is still to track the runner-images repo itself.