Automatic merge from submit-queue (batch tested with PRs 41357, 41178, 41280, 41184, 41278)

Switch RBAC subject apiVersion to apiGroup in v1beta1

Referencing a subject from an RBAC role binding, the API group and kind of the subject is needed to fully-qualify the reference.

The version is not, and adds complexity around re-writing the reference when returning the binding from different versions of the API, and when reconciling subjects.

This PR:
* v1beta1: change the subject `apiVersion` field to `apiGroup` (to match roleRef)
* v1alpha1: convert apiVersion to apiGroup for backwards compatibility
* all versions: add defaulting for the three allowed subject kinds
* all versions: add validation to the field so we can count on the data in etcd being good until we decide to relax the apiGroup restriction

```release-note
RBAC `v1beta1` RoleBinding/ClusterRoleBinding subjects changed `apiVersion` to `apiGroup` to fully-qualify a subject. ServiceAccount subjects default to an apiGroup of `""`, User and Group subjects default to an apiGroup of `"rbac.authorization.k8s.io"`.
```

@deads2k @kubernetes/sig-auth-api-reviews @kubernetes/sig-auth-pr-reviews