[PureOS] Byzantium versioning

Hi, since we're dropping phone specific package for byzantium we also ought to drop `+librem5.X' in the version numbers. Matthias, what would you suggest here so that we generate higher version numbers in byzantium when backporting a package to both amber and byzantium to be consistent whith the rest of PureOS but also have a higher number than the current

<upstream>-<debian-rev)pureos+librem5.<revision>

Cheers, -- Guido