Automatic merge from submit-queue (batch tested with PRs 51956, 50708)

Move autoscaling/v2 from alpha1 to beta1

This graduates autoscaling/v2alpha1 to autoscaling/v2beta1.  The move is more-or-less just a straightforward rename.

Part of kubernetes/features#117

```release-note
v2 of the autoscaling API group, including improvements to the HorizontalPodAutoscaler, has moved from alpha1 to beta1.
```