Migrate HorizontalPodAutoscaler to autoscaling/v2

Move HorizontalPodAutoscaler off the autoscaling beta APIs — v2beta1, removed in Kubernetes v1.25, and v2beta2, removed in v1.26 — onto autoscaling/v2, restructuring the metric targets v2beta1 wrote inline.

v2beta2 is autoscaling/v2 under another name, so those resources only change version. A v2beta1 resource is rewritten the way the apiserver itself converted it: targetAverageUtilization becomes target.averageUtilization under target.type: Utilization and targetAverageValue becomes target.averageValue under target.type: AverageValue; metricName and its selector become the metric identifier; and for an object metric the target naming the described object becomes describedObject, freeing target for the metric target built from targetValue and averageValue.

A metric that sets a combination autoscaling/v2 validation rejects — a resource metric targeting both a utilization and a raw value, or an external metric targeting both a total and a per-pod value — is left on its old API version and marked with the reason.