gbp.conf: Drop 'v' from version numbers

This is more in line with what other packages do.
4 jobs for pureos/byzantium in 9 minutes and 38 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Package
passed #327200
librem5
package:deb-debian-bullseye

00:03:35

passed #327201
librem5:arm64
package:deb-debian-bullseye:arm64

00:07:40

 
  Test Package
passed #327202
librem5
autopkgtest-debian-bullseye-package

00:00:49

passed #327203
librem5
lintian-debian-bullseye-package

00:00:57