PAASTA-18383: Update V2Beta2 HPA api references to V2 for 1.26

[jfongatyelp]

As per https://kubernetes.io/docs/reference/using-api/deprecation-guide/#horizontalpodautoscaler-v126 the V2beta2 version of the HorizontalPodAutoscaler api is being deprecated in 1.26.

This updates as per recommendations to v2, and as all of our clusters are >1.23 they should support it.
Note that we did not use targetAverageUtilization that has been switched to target.type=Utilization+target.averageUtilization, as we'd already used the latter in our code.

This has been tested in paasta playground and infrastage and s_k_j was able to create/modify the appropriate HPA objects and autoscaling was successful.

Confirmed with audit logs that paasta was sending the request to autoscaling/v2 rather than autoscaling/v2beta2 with the change.