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