CI on macOS 15

[EliahKagan]

macos-latest currently aliases macos-14. As noted in this official GitHub blog post, it shall be migrated to macos-15 next month. This is why workflow runs that use macos-latest now show informational annotations saying:

The macos-latest label will migrate to macOS 15 beginning August 4, 2025. For more information see actions/runner-images#12520

Everything should be fine and we shouldn't have to do anything. I say this because I have tested replacing macos-latest with macos-15, to achieve the near-future effect now, on a branch of my fork, and everything works. There are two workflows that contain jobs that use macos-latest: the ci.yml and release.yml. Both work with macOS 15.

Since the release workflow is one of the ones that made sense to test--actually, it was more important to test it, since a problem in ci.yml would likely be noticed before becoming especially cumbersome--it was convenient to test this while working on #1970. To clarify, replacing macos-latest with macos-15 is just for testing; we do not need to integrate that change, and I did not include it in #2077.

(It would be strange to open an issue to say something is working and nothing has to be done, so I'm posting this as a discussion. I'm making it a question so I can take advantage of the "answered" status to represent an intermediate state of resolution--I plan to close it once the migration has actually occurred, but it can be marked answered almost immediately. However, this also has the benefit that if you're aware of any Sequioa-related problems or anything else that should be tested, then you can let me know.)