GitHub Actions: Use oldest Ubuntu and newest macOS

[arichardson]

This ensures that we can build on macOS 11.0
and the oldest supported Ubuntu release
(both of which were previously broken).

[jrtc27]

11 not 11.0 (now following an iOS-like versioning scheme)

[arichardson]

Looks like the GitHub documentation is wrong then..

[jrtc27]

Hm, actions/runner-images#2486. So the name is right, but it's a terrible one done without any proper thought.

[jrtc27]

I think we want to keep the existing jobs? Certainly keeping a non-ancient Ubuntu would be good otherwise we risk regressing support for more modern glibcs.

[arichardson]

I think we want to keep the existing jobs? Certainly keeping a non-ancient Ubuntu would be good otherwise we risk regressing support for more modern glibcs.

I agree we should but that's handled by our Jenkins tasks.

[jrtc27]

I think we want to keep the existing jobs? Certainly keeping a non-ancient Ubuntu would be good otherwise we risk regressing support for more modern glibcs.

I agree we should but that's handled by our Jenkins tasks.

Not upstream.

[arichardson]

I think we want to keep the existing jobs? Certainly keeping a non-ancient Ubuntu would be good otherwise we risk regressing support for more modern glibcs.

I agree we should but that's handled by our Jenkins tasks.

Not upstream.

Yeah this is only for CheriBSD. We've already got different jobs.

[jrtc27]

I think we want to keep the existing jobs? Certainly keeping a non-ancient Ubuntu would be good otherwise we risk regressing support for more modern glibcs.

I agree we should but that's handled by our Jenkins tasks.

Not upstream.

Yeah this is only for CheriBSD. We've already got different jobs.

Only minor divergence currently?

[arichardson]

Submitted upstream as https://reviews.freebsd.org/D28322. Will drop the macOS change since it doesn't work yet.