debian-13 template

[How to file a helpful issue](https://www.qubes-os.org/doc/issue-tracking/)

### The problem you're addressing (if any)

Make it possible to test stuff on Debian testing (trixie).

### The solution you'd like

Prepare packages and (maybe) pre-built template for Debian 13.

### The value to a user, and who that user might be

Make stuff ready well before final Debian 13 release.
