[eks] [request]: Better documentation for kube-proxy version

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**Tell us about your request**
Better documentation for which version(s) of `kube-proxy` are supported.

**Which service(s) is this request for?**
EKS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
The `kube-proxy` version in the [EKS Upgrade Guide](https://docs.aws.amazon.com/eks/latest/userguide/update-cluster.html) doesn't line up with the K8s version used in the [AMI releases](https://github.com/awslabs/amazon-eks-ami/releases) (and has regressed for v1.18, v1.17 & v1.16) or the versions available in the ECR public container registry.

**Are you currently working around this issue?**
n/a

**Additional context**
n/a

**Attachments**
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Compare the kube-proxy versions in the EKS Upgrade Guide with the Kubernetes versions in the amazon-eks-ami releases and the versions available in the ECR public container registry. Done means the supported kube-proxy versions are documented consistently, including the noted v1.16, v1.17, and v1.18 regressions.

Written by the indexing model from the issue text.