Stronger recommendations re backups

Related: https://forum.getodk.org/t/cant-login-after-upgrading-central-to-v2024-1/48290/15

We need to have stronger language in the post-install of DO and other places that people must have full system backups and upgrade regularly.

And if they don't know what that means, we should tell them to run Central on infrastructure where they can regularly snapshot the entire machine and restore to a clone so they can test upgrades.

Start with the post-install guidance for DO and review the linked forum discussion for the failure context. Identify the other places that discuss upgrades, then update the guidance so backup, regular-upgrade, and full-machine snapshot expectations are clear; done means the post-install and related guidance consistently communicate the recommendation.

Written by the indexing model from the issue text.