Upgrade Vagrant basebox to Bookworm
This is part of our usual KTLO.
Please note that we don't even need to do it: we can keep the Bullseye one until Debian 13 is released. We should at some point switch to LTS security support, but it might be the case (to be investigated) that this time we don't actually need to do anything. That's the reason this is not in the roadmap.