Automatic merge from submit-queue

Update Stateful Set example files for 1.5

1. Remove initialized annotation from statefulset examples
2. Update storage class annotation to beta in statefulset examples
3. Remove alpha limitation on PetSet in cassandra example

cc @erictune @foxish @kow3ns @enisoc @chrislovecnm @kubernetes/sig-apps

```release-note
NONE
```