CI setup will go out-of-date

[dabrahams]

We have jobs in our actions that are titled with particular compilers, etc. However, we're really not testing those compilers unless that happens to be the current latest setup at GitHub. For example, we say “Build and Tests / macOS apple-clang 13.0.1” whereas the latest xcode ships with apple clang version 13.1.6. That's almost certainly what GH is using right now, but if not, they will be soon enough.

We need to decide whether we mean to say certain compiler versions are supported/tested, or whether we're going to be playing catch-up to whatever GitHub has as their latest configuration. I recommend the former, since GH doesn't re-run all actions when they change default tester configuration.