control: Drop version constraints

If we consider the latest Debian stable release (Debian 11 bullseye) and the latest Ubuntu LTS release (Ubuntu 20.04 LTS focal) to be the only sensible targets for a potential libvirt backport, we can simplify the control file quite a bit.

@agx and I agree on going in this direction to keep things maintainable (see the original discussion for more details), but we would also like to make sure that doing so will not make @paelzer-guest's life miserable :)