Update .ci.yaml in flutter/packages to use either macOS 14 or macOS 15 #167825
Description
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
- os: Mac-14
+ os: Mac-14|Mac-15