Move the MPS M1 test shard off macOS 14 (#199118)

The us-east-1 mac2 fleet has finished migrating to Sequoia, so no `macos-m1-14` runner exists anymore and this shard would queue forever.

Points the M1 MPS shard at `macos-m1-15` and swaps the label in `actionlint.yaml` (`macos-m1-15` was not in the allowed list yet).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/199118
Approved by: https://github.com/jeanschmidt