protect branches against cross-version pushes

last weekend a 10.5 merge was mistakenly pushed into 10.4. To fix that I had to force-push into the main 10.4 branch. This is a very expensive fix and it would be best if branch protection would prevent such pushes in the future.

To this it buildbot needs to ensure that a 10.5 commit, won't be allowed into 10.4. To do it buildbot can provide a set of dummy builders that don't build anything but only look at the VERSION file and immediately return whether it matched the builder name. That is, builder names are, like, ok-10.3, ok-10.4, ok-10.5, etc. Then any bb-10.5-merge push will pass ok-10.5, but will fail ok-10.4, and the latter builder will be required for any push into 10.4