Automatic merge from submit-queue (batch tested with PRs 61930, 61885). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Fix comment in CRI run_as_group.

This PR:
1) Clarify that runtime should return error if `run_as_group` is set when `run_as_user` is not. (Current behavior in dockershim, https://github.com/kubernetes/kubernetes/blob/master/pkg/kubelet/dockershim/security_context.go#L93)
2) Remove `run_as_groupname` which is not supported now.

/cc @kubernetes/sig-node-api-reviews 
/cc @krmayankk 
**Release note**:

```release-note
none
```