update macOS ci image

[timabell]

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

The macOS 13 runner image will be retired by December 4th, 2025

[timabell]

might be worth doing this at the same time

On macOS, we can set MACOSX_DEPLOYMENT_TARGET to 10.7 to only depend on system features present in versions 10.7 and older.
~ https://rust-cli.github.io/book/tutorial/packaging.html#building-binary-releases-on-ci