Move Autoscaling v2{alpha1 --> beta1}
Move Autoscaling v2{alpha1 --> beta1}

This commit renames autoscaling/v2alpha1 to autoscaling/v2beta1.
Only the API-related code is moved in this commit.

Kubernetes-commit: b0af4024753bd0eac43a57282b3f6414488db299

kubernetes/code-generator,kubernetes/code-generator