macOS runners do not meet minimum macOS SDK requirements for mozjs

[jdm]

https://github.com/servo/servo/actions/runs/16567598331/job/46851363666

ERROR: SDK version "13.3" is too old. Please upgrade to at least 14.2. Try updating your system Xcode.

I think there's an interesting sequence that leads to this:

• the vast majority of macOS builds on CI can use the prebuilt mozjs binaries
• the vast majority of try builds that use a non-default mozjs revision are performed using non-macOS try targets
• doing a full try build that forces a mozjs build from source hits this macOS error

This can be worked around for development purposes by including a temporary commit that replaces https://github.com/servo/servo/blob/8008d5aa8573a25bda53bdd372241d54ccca69da/.github/workflows/mac.yml#L93C7-L93C33 with force-github-hosted-runner: true since the github runners have an updated SDK.

[delan]

i can’t find a clear answer on whether (or how) we can install the newer sdk (14.2) on macOS 13, but we may be able to get away with just upgrading our images to macOS 14 or 15.