Run tests on macOS 15.5 or 15.7

[okorohelijah]

In preparation of upgrading our fleet to macOS 15.7, allow tests to run on either macOS 15.5 or 15.7.

List which issues are fixed by this PR. You must list at least one issue.
Fixes flutter/flutter#176931

Pre-Review Checklist

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[gemini-code-assist]

Code Review

This pull request updates the CI configuration in .ci.yaml to modify the macOS versions for testing. It replaces Mac-14 with Mac-15.7, setting the target operating systems to Mac-15.5 or Mac-15.7. My review highlights that this change removes Mac-14 from the test matrix, which could create a gap in test coverage. I've suggested retaining Mac-14 to ensure continued support while adding the new version.

[gemini-code-assist]

This change removes Mac-14 from the list of operating systems for this test configuration. This could result in an unintended gap in test coverage for macOS 14. To ensure continued testing on the older OS while adding support for Mac-15.7, consider including all three versions.

      os: Mac-14|Mac-15.5|Mac-15.7

[gemini-code-assist]

Similar to the change above, this removes Mac-14 from the test matrix for x86 CPUs. To maintain test coverage on macOS 14, it is advisable to add Mac-15.7 without removing the existing OS version.

      os: Mac-14|Mac-15.5|Mac-15.7