Update our macOS CI

[ISSOtm]

actions/runner-images#13518 says that our use of macOS 14 in CI is to come to a close soon. We can upgrade merely to macos-15, but we now also have the option to use macOS 26, and both can be used on both ARM and Intel. The question really is how many CI configurations we want to have. More = better? Not quite: the more configs we have, the higher it counts against the concurrency cap, and generally isn't very parsimonious.

My personal opinion: it's fine to only test on a single arch each (perhaps 15-intel and 26 ARM?), since we don't employ anything architecture-specific.

[Rangi42]

"Earliest available Intel" and "latest available ARM" sounds good to me.