Disable noble migration at end of June 2025

[legoktm]

Focal will go end-of-life at the end of May, at which point the SI will disable itself. We'll give people a 1-month grace period to use the noble migration to upgrade, but after that we'll disable it because:

1. We don't want servers to sit without security updates for a long time and then go back into usage.
2. The noble migration code is liable to break in the future since noble is a bit of a moving target, and it will no longer be actively tested.

Admins will still be able to manually upgrade by taking a backup and restoring it on a fresh install.

Note that we'll need to push focal and noble packages for this.