A mailing list's last_post_at is not updated.

This is supposed to be done by mailman/handlers/after_delivery.py, but that handler sets mlist.last_post_time which is the Mailman 2.1 name of the attribute which is now named last_post_at in the mailing list model.

The test in mailman/handlers/docs/after-delivery.rst doesn't catch this because it too references last_post_time which is set as a non-persistent attribute of the test list.