text: Title: Move selector immutability check to validation after v1beta1 retires
  Body: The check for selector immutability is located at `PrepareForUpdate` functions of `deploymentStrategy`, `rsStrategy` and `daemonSetStrategy`. We are not able to have the check at validation before _v1beta1_ API version retires due to breaking change to some tests (discussed in this closed [PR](https://github.com/kubernetes/kubernetes/pull/50348)). Once _v1beta1_ retires, we should move the check to validation.
  
  