docs: explain addon version differences

[atharvaHJoshi]

Fixes #7864

What this PR does

Users compared the version applied by eksctl utils update-aws-node /
update-kube-proxy / update-coredns against the list from
eksctl utils describe-addon-versions and saw mismatches. This is by design, but the
docs did not explain it.

Changes

Adds a "What 'latest' means for each command" section to the default add-on update
guide that clarifies:

• describe-addon-versions lists EKS Managed Add-on versions (their own release
  cadence and -eksbuild.x suffix).
• update-aws-node / update-coredns apply the manifest bundled with the eksctl
  release ("known good" version), not the newest EKS-managed version.
• update-kube-proxy resolves the newest version for the cluster's control-plane
  version from the EKS API.
• How to get the newest EKS-managed version instead (install/update them as managed
  add-ons via eksctl create addon / eksctl update addon --version latest).

[github-actions]

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