Automatic merge from submit-queue. 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>.
Migrate FeatureGates type of kube-proxy from string to map[string]bool
**What this PR does / why we need it**:
Migration of FeatureGates type. This is a follow-up of #53025.
**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: #53025https://github.com/kubernetes/kubernetes/pull/57754#discussion_r160023416
**Special notes for your reviewer**:
/cc @luxas @mtaufen @ncdc
**Release note**:
```release-note
action required: kube-proxy: feature gates are now specified as a map when provided via a JSON or YAML KubeProxyConfiguration, rather than as a string of key-value pairs.
```
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