macOS-13 GA runner being deprecated

This just showed up today:

https://github.com/DynamoRIO/dynamorio/actions/runs/19472984992/job/55730594854?pr=7715

> This is a scheduled macos-13 brownout. The macOS-13 based runner images are being deprecated. For more details, see https://github.com/actions/runner-images/issues/13046.