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Add note on upgrading cluster by kubeadm.

**What this PR does / why we need it**:
Upgrading cluster from 1.9.x to 1.10.0 by kubeadm fails due to type change of `featureGates` in `KubeProxyConfiguration` done in #57962. This PR add a note on what should be done before upgrading.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
ref #61764

**Special notes for your reviewer**:
cc @kubernetes/sig-cluster-lifecycle-bugs @liggitt @fgbreel
Thanks @berlinsaint for the workaround!

We may need a patch (if possible) for kubeadm to do this automatically as suggested by @danderson .

**Release note**:

```release-note
NONE
```