GitHub Actions: macOS 13 runner image is closing down

[cclauss]

Replace GitHub Actions runner image macos-13 with macos-15-intel as recommended in:

• https://github.blog/changelog/2025-09-19-github-actions-macos-13-runner-image-is-closing-down

The currently available GitHub Actions macOS runners are:

macOS Version	runner.arch
macos-13	X64
macos-14	ARM64
macos-15	ARM64
macos-15-intel	X64
macos-26	ARM64
macos-latest	ARM64

[mattip]

We can also run the macos-arm64 CI on macos-15, which is native arm64

[cclauss]

The choice between macos-{14,15,26,latest} is a bit more nuanced because none of them are deprecated by Apple or setup-python. Apple usually supports three versions of macOS. I would pick macos-latest (currently 15) on a repo is not very active to take advantage of setup-python’s effective rolling releases. There would also be a strong argument for jumping to macos-26 as other projects have discovered compatibility issues in doing so and macOS users tend to upgrade faster than users of other operating systems.