Replace GHA `macos-13` runner images before end of deprecation

[elle-j]

Some of our CI workflows are using the macos-13 image which will be fully unsupported by early December, 2025.

"Workflows using the macos-13 image label will be terminated with an error."

Available replacements: macos-latest, macos-15, or macos-14.

For x86_64 Intel environments, there is a new label macos-15-intel available until Aug. 2027.

[xermicus]

Nice find, thanks! Would you be up to taking care of this? I.e. switching to macos-15 (using the newest versions is fine but please just no latest runners, I don't like when they get eventually upgraded and this may break the CI without manual control).

[elle-j]

No problem, I'll jump on it next 👍