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Kubelet: only sync iptables on linux

**What this PR does / why we need it**:

Iptables is only supported on Linux, kubelet should only sync NAT rules on Linux.

Without this PR, Kubelet on Windows would logs following errors on each `syncNetworkUtil()`:

```
kubelet.err.log:4692:E0711 22:03:42.103939    2872 kubelet_network.go:102] Failed to ensure that nat chain KUBE-MARK-DROP exists: error creating chain "KUBE-MARK-DROP": executable file
```

**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 #65713

**Special notes for your reviewer**:

**Release note**:

```release-note
Kubelet now only sync iptables on Linux.
```