Resolve #3587543 "Forever end phpstan ci disruptions"

See https://www.drupal.org/project/canvas/issues/3587543 for rationale.

New CI job that fails (and is allowed to) if >=1 of our PHPStan packages is outdated:

When it is up-to-date:

Bonus: no longer running on every commit, only during scheduled pipelines — i.e. nightly:

Closes #3587543