Stronger recommendations re backups

[yanokwa]

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.