Support macOS 15

[outofambit]

While investigating w3c/aria-at#1175, I realized that our automation doesn't actually support macOS 15 (Sequoia).

Unfortunately, the GitHub Actions runner image for macOS 15 doesn't currently support setting VoiceOver settings via the command line as we do in our setup. That problem is tracked at actions/runner-images#11257. We are mostly blocked on this until that is resolved.