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

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[gemini-code-assist]

Code Review

This pull request updates the CI configuration to run tests on macOS 15.5 or 15.7, replacing the previous configuration that targeted macOS 14 and 15.5. My review suggests that during this transition, it might be safer to temporarily continue testing on macOS 14 to prevent any potential regressions. I've added comments with suggestions to include all three versions in the test matrix for both arm64 and x86 architectures.

[gemini-code-assist]

This change removes Mac-14 from the list of test platforms for arm64. To prevent potential regressions on Mac-14 during the transition to Mac-15.7, it would be safer to test against all three versions temporarily.

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

[gemini-code-assist]

This change removes Mac-14 from the list of test platforms for x86. To prevent potential regressions on Mac-14 during the transition to Mac-15.7, it would be safer to test against all three versions temporarily.

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

[vashworth]

@okorohelijah Why was this closed?

[okorohelijah]

@vashworth nit: i created the pr directly from my main