Migrate to Debian 12 Bookworm

Previously: Migrate to Debian 11 (#516 - closed)

This will make the wikis run on PHP 8, which will be needed soon now that MediaWiki 1.42 has officially dropped support for PHP 7.4 (even though we still run it at WMF).

Bonus planned changes to the setup:

• Set up the new instance in the catalyst project rather than visualeditor
• Redirect from https://patchdemo.wmflabs.org to https://patchdemo.wmcloud.org