CI matrix update: add Ubuntu 21.04 - second attempt

[aik-jahoda]

The original CI matrix change was reverted (#60040) because of a failing CI.

part of #57947

original PR: #59578

[ViktorHofer]

Same comment as in your other PR:

I didn't check the yml. Does this added queue only run as part of a rolling build? If so, can you please queue up a build with your changes to make sure that the changes work as expected?

[hoyosjs]

Same comment as in your other PR:

I didn't check the yml. Does this added queue only run as part of a rolling build? If so, can you please queue up a build with your changes to make sure that the changes work as expected?

As it stands now - it would only run in a rolling build

[karelz]

@safern @ericstj can you please review the PR? Then we can merge (and backport to .NET 6.0). Thanks!

[ericstj]

I believe there is an open question from reviewers above which requires attention:

Does this added queue only run as part of a rolling build? If so, can you please queue up a build with your changes to make sure that the changes work as expected?

As it stands now - it would only run in a rolling build

@aik-jahoda Can you share the results of the rolling build which tests your changes?

[aik-jahoda]

Test result: https://dnceng.visualstudio.com/public/_build/results?buildId=1493975&view=logs&j=b9b7de4d-2126-5581-6453-1576f9aaac5f&t=db048d48-40e8-5399-9b14-d587763f33ff&l=38

[aik-jahoda]

@safern, @ericstj , can you please take a look?

[ericstj]

This looks good to me. @safern do you agree?

[ericstj]

Actually, I'm not entirely certain how to interpret all these failures in the other legs. I see the linked log shows at least one pass on the new queues. Do you understand the other failures?

[safern]

LGTM, thanks.