Update of buster64 and contrib-buster64 Vagrant images

[Lucas Nussbaum <lucas@debian.org>]

Hi,

The Vagrant images for buster64 and contrib-buster64 have just been
updated. The building process now uses the debian-vagrant-images git
repo[1], which is based on FAI and on the debian-cloud-images repository
that is used for cloud images. (There is still work to do before we can
fully integrate the Vagrant-specific bits into debian-cloud-images).

[1] https://salsa.debian.org/cloud-team/debian-vagrant-images/

Since the build process is different from the previous packer-based one,
it is possible that some differences have been introduced with this
update.  Please report regressions as bugs, so that we can track them
and possibly fix them.

In addition to those images, the testing64 image was also updated, and a
bullseye64 image was also added to prepare for the bullseye release.

Lucas