Since Debian Buster can not be used with nova 23.0.0 because of the
min required libvirt version, we should make Bullseye available for CI
to ensure that OpenStack Wallaby release will run on it smoothly.
Depends-On: https://review.opendev.org/c/openstack/diskimage-builder/+/783790
Change-Id: I9c1bb7aaa02ba60ee52e2d7b990e2e6e1212317f