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Fix kubeadm upgrade diff -config TODO
**What this PR does / why we need it**:
This PR addresses a TODO in `kubeadm upgrade diff` for the `-config` flag to use `options.AddConfigFlag()` instead of `cmd.Flags().StringVar()`.
**Which issue(s) this PR fixes**
None
**Special notes for your reviewer**:
/sig kubernetes/sig-cluster-lifecycle-pr-reviews
**Release note**:
```release-note
NONE
```
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