Simplify versioning process

While discussing gitlab-workhorse versioning with @nick.thomas we came up with a simpler rule for when to create a minor or a patch version. The rule deviates from SemVer intentionally, in order to make it easier to meet the quality standards of GitLab stable branches.

The motivating example is that SemVer says that a release that only contains a refactoring should only increase the patch counter, while from the point of view of GitLab stable branches, it is not right to back-port a refactoring (a pure refactoring that is not known to fix bugs) into a stable branch.

Workhorse process MR: gitlab-workhorse!334 (merged)