[PureOS] Byzantium versioning

[Guido Günther]

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