Automatic merge from submit-queue (batch tested with PRs 52355, 52537, 52551, 52403, 50673). 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>..
Drop --experimental-bootstrap-token-auth flag.
**What this PR does / why we need it**:
Drops support for the `--experimental-bootstrap-token-auth` flag for the API server. This flag was replaced by `--enable-bootstrap-token-auth` in 1.8 (https://github.com/kubernetes/kubernetes/pull/51198) but the old flag was accepted with a deprecation warning. In 1.9 we want to totally drop it.
**Which issue this PR fixes**: fixes#50613
**Special notes for your reviewer**:
There are other places in kubeadm where we need to drop support for this flag (at init and at postupgrade). We can tackle those in a separate PR.
**Release note**:
I think this change is already document well enough in the changelog for 1.8.
```release-note
NONE
```
@kubernetes/sig-cluster-lifecycle-pr-reviews @kubernetes/sig-auth-pr-reviews
/kind cleanup
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