Add PHP 8.1 as highest supported version for 401_STABLE and master
As far as https://tracker.moodle.org/browse/MDL-73016 is almost done and we have been getting stable results in core, this just adds the PHP 8.1 version officially to 401 and master.
That implies that all jobs (but mariadb ones, that always use the lowest supported version) will start using the new highest (PHP 8.1) automatically.
Agreed with Workplace, we aren't adding that to the WP and WPR jobs yet, because they need their branches rebased on top of upstream versions yet. So, those WP* versions will be updated then.