buildserver: turn ./makebuildserver into a pure helper script, allow standalone vagrant operation

Up until now, the whole ./makebuildserver process required the makebuildserver script to work. This merge request moves the config to buildserver/Vagrantfile, purges some unmaintained features, and canonicalizes the fdroid/bullseye64 base box. If someone wants to change the base box, e.g. for dev, then they can just edit buildserver/Vagrantfile directly. This process also does not automatically upgrade existing boxes, so ./makebuildserver can still be used for that, or via the normal vagrant box ... commands.

This hopefully will contribute to ultimately getting rid of makebuildserver entirely, and just using Vagrant itself. The box verification stuff really belongs there, for example.