Update ci.yaml in flutter/cocoon to use either macOS 15.5 or 15.7.1

[okorohelijah]

Update ci.yaml in flutter/cocoon to use 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#179057

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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