chore(ci): use macos-15-intel for x86 wheel building and testing

[ariostas]

The wheels that are built in macos-14 and macos-15 are identical (since they are both arm64), so I just switched to using `macos-latest.

This closes #3718 which was failing since there were duplicate wheels.

Also, I guess we quietly dropped support for x86 macs, but no one has complained, so I guess it's fine.

[ikrommyd]

Also, I guess we quietly dropped support for x86 macs, but no one has complained, so I guess it's fine.

I see x86 macos wheels under https://pypi.org/project/awkward-cpp/50/#files

[ariostas]

Oh right, there hasn't been an awkward-cpp release since #3666, so I guess we still have time to properly announce it and/or bump minor version when there is a new awkward-cpp release.

[github-actions]

The documentation preview is ready to be viewed at http://preview.awkward-array.org.s3-website.us-east-1.amazonaws.com/PR3720

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.71%. Comparing base (b749e49) to head (d6a34a7).
⚠️ Report is 467 commits behind head on main.

Additional details and impacted files

see 200 files with indirect coverage changes

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• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[ariostas]

Thanks to Henry for pointing out that macos-15-intel was an option!