gbp: Rename Debian branch

Andrea Bolognanirequested to merge
abologna/libvirt-dbus:rename-debian-branch into debian/latest

The current version of DEP14 (CANDIDATE status, updated 2020-11-29) recommends using debian/latest for development.

We have been following a similar naming scheme for the libvirt package and it's proven to be quite nice to work with, so adopt it for libvirt-dbus too.

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