Update .ci.yaml in flutter/packages to use either macOS 14 or macOS 15

[vashworth]

Prerequisites

• Run all flutter/packages macOS tests manually on macOS 15.5+ bot #167822
• Ensure no open issues in Tracking issue for any infra or test failures with macOS 15 #167681 under flutter/packages

---

https://github.com/flutter/packages/blob/main/.ci.yaml

- os: Mac-14
+ os: Mac-14|Mac-15

[github-actions]

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