Switch CI to `macos-12` to fix failures

GitHub hosted runners don't yet support macOS Monterey, which is the cause of current CI failures. We should update this as soon as it becomes available so we can add tests and fix the failing lint workflow.

Tracking Issue: https://github.com/github/roadmap/issues/240

Inspect the repository's CI configuration and the failing lint workflow, then verify whether GitHub-hosted runners support macos-12 as described in the issue and Tracking Issue. Done means the workflow uses macos-12 and the CI failures are unblocked; further test or lint fixes are outside this issue's stated change.

索引モデルが issue の本文から書いたものです。