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kube-proxy: fix field name comments & json tags

**What this PR does / why we need it**: correct some minor issues in the comments and json tags for some of the fields in the kube-proxy config structs.

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

**Special notes for your reviewer**:

**Release note**:

```release-note
The udpTimeoutMilliseconds field in the kube-proxy configuration file has been renamed to udpIdleTimeout. Action required: administrators need to update their files accordingly.
```

This was extracted from my currently unmerged https://github.com/kubernetes/kubernetes/pull/52198/commits/f074b28fe9cae52871c76e2834e70975ab473f7e, as requested [here](https://github.com/kubernetes/kubernetes/pull/52198#pullrequestreview-85538637).

@kubernetes/sig-network-pr-reviews @luxas