Update GHA for mac.

[1uc]

We're getting the following warning from GHA:

HighFive_CI: .github#L1
The macOS-10.15 environment is deprecated, consider switching to macos-11(macos-latest), macos-12 instead. For more details see actions/runner-images#5583

We'll likely need to slightly modernize our CI for mac.

[tdegeus]

I think we should just run with latest

[1uc]

The downside of that is that that then our CI breaks whenever latest changes. When that happens fixing CI is the number one and only priority and will prevent merging anything else.

Personally, I'm not very keen on that type of setup, since I fear Murphy's law, i.e. it'll break when we need to merge something urgently, but don't have the time to fix CI for a new OS. (We'd have time a few weeks later or before, just not right then.)

I seem to remember that for a certain period of time for Ubuntu latest could mean "any of two choices". Which doesn't work well if you need to use slightly different commands to install dependencies.

OTOH, it would prevent us form running 2 major versions behind.

[tdegeus]

Things should not break if latest changes ;) And if they do we should know soon as possible, as users will also be updating.

I would not worry however that it would become a blocker. If this happens we can always quickly downgrade the CI on order to still have passing checks. In addition, that is a special case anyway that needs special attention anyway.

My personal strategy is to only hardcode when needed, not to hardcode when possibly useful.

But, whatever you want ;).

[pramodk]

The downside of that is that then our CI breaks whenever latest changes.

Don't have a strong opinion but have similar experiences with some other projects. Especially (if/) when rely on some of the stuff "custom sauce or updates" that GitHub and/or Azure actions runners are shipped with.