The next release should be 6.x-1.13, skip 1.12 entirely

To help avoid confusion I think we should release 6.x-1.13 next, skipping 1.12 entirely. Once we re-roll more fixes from the old 6.x-1.12-beta timeline I think we'll be in a good position to release 6.x-1.13-beta1, then 6.x-1.13-rc1, then the first solid release in nearly two years!