Automatic merge from submit-queue (batch tested with PRs 51494, 56097, 56072, 56175). 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>.

oidc auth: fix prefix flag plumbing

Fixes https://github.com/kubernetes/kubernetes/issues/56169

Tested this with an full API server against Google. It works now.

:(

cc @kubernetes/sig-auth-bugs 

```release-note
kube-apiserver: fixed --oidc-username-prefix and --oidc-group-prefix flags which previously weren't correctly enabled
```

/assign @liggitt @deads2k 

Probably worth a cherry pick.