version: use next version ~rc for unstable branch
version: use next version ~rc for unstable branch

This makes unstable always sort after stable, testing releases:
  0.21~rc-unstable... > 0.20.1-testing...

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph