Check out previous tag after restoring direct backup after failed upgrade

We emphasize backing up the server before upgrading Central, but for direct backups, I think there's a step missing about restoring from backup. After a failed upgrade, even if the user restores the database from a direct backup, they will still be at the tip of the `master` branch in Git. Unless they check out a different commit, I think rebuilding Central will retry the upgrade and attempt to run the latest migrations.

Locate the documentation covering direct backups, failed upgrades, and restoring the database. Read the surrounding upgrade and rebuild instructions first, then add the missing guidance to check out the previous tag after restoring a direct backup; done means the procedure prevents rebuilding Central from retrying the latest migrations.

Written by the indexing model from the issue text.