Consider toggling maintenance mode during pg-upgrade rather than the deploy page

Summary

Currently our pg-upgrade script has a step for enabling the deploy page. This is really only meaningful on a single node install. But a better alternative, that may also make sense for other install types, is to toggle maintenance mode: https://docs.gitlab.com/ee/administration/maintenance_mode/#enable-maintenance-mode

We should take a closer look, and at least consider having it added as a suggested manual step to the documentation for when we aren't auto-upgrading the database.