Move to monobranch release strategy

Might be some disagreement on this - but I believe we'd be better able to serve the users by consolidating to a single main branch that we release to all live distributions. That way we can skip the "backport ballet" necessary to give new features to e.g. Jazzy & Humble.

This comes with some increased rigidity for API stability, but I think we can work around that with deprecations as needed.