Automatic merge from submit-queue (batch tested with PRs 51733, 51838)
Relax update validation of uninitialized pod
Split from https://github.com/kubernetes/kubernetes/pull/50344
Fix https://github.com/kubernetes/kubernetes/issues/47837
* Let the podStrategy to only call `validation.ValidatePod()` if the old pod is not initialized, so fields are mutable.
* Let the podStatusStrategy refuse updates if the old pod is not initialized.
cc @smarterclayton
```release-note
Pod spec is mutable when the pod is uninitialized. The apiserver requires the pod spec to be valid even if it's uninitialized. Updating the status field of uninitialized pods is invalid.
```
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