Modify version.xml to support vRELEASE tags

This commit should add support for tags prefixed with v (as opposed to v. previously)

To test it:

- [ ] check the merge build (using the existing `v.5.1.0-m0.3` tag)
- [ ] create an annotated  `vx.y.z` tag on a previous commit
- [ ] create an annotated  `vx.y.z` tag on the current commit

In all scenarios
- make sure `./build.py version-info` returns the correct version (i.e. uses the latest tag)
- build the server/clients and check the version number are correct