Remove 'v' prefix from Debian tags
Don't add an extra 'v' to tags as it violates DEP-14 guidance and breaks tools like git-buildpackage and git-debpush from reasoning correctly about what the tags are.
In practice this will make:
- debian/v2.11.0-2 -> debian/2.11.0-2
- upstream/2.11.0 -> upstream/2.11.0
Only the upstream tag can or should have 'v' in it, but it is currently not defined at all in gbp.conf as the field upstream-vcs-tag is missing.