GKE cniBinDir documentation removed

[howardjohn]

https://github.com/istio/istio/issues/53849#issuecomment-2520841621 -- refactoring removed any documentation of cniBinDir setting for GKE. https://istio.io/latest/docs/ambient/install/platform-prerequisites/#google-kubernetes-engine-gke is missing it and https://istio.io/latest/docs/setup/additional-setup/cni/ links only there

cc @bleggett

[bleggett]

This is already fixed (the right way) in https://github.com/istio/istio.io/pull/16035 but wasn't backported.

I'll do a 1.24 only tweak docs backport that will be superceded in the next release with the platform profile anyway - that will automatically also cover this case (which only happens today if you aren't using either of the two recommended install methods of istioctl or helm)

[bleggett]

There are probably other template-install-only gotchas we have, but we do not and have not ever really consistently documented them.

[gojanpaolo]

we encountered a related issue where cni pods throws permission denied on the default cniBinDir /opt/cni/bin. we use gke and argocd (argocd uses helm template) setting the cni.cniBinDir to /home/kubernetes/bin fixed the issue

side question: why do we need to set cni.cniBinDir, instead of just cniBinDir, when using helm template on the cni chart? also, I think adding that as a comment right beside the cniBinDir helm values would help. e.g.

$ helm show values istio/cni
...
  # set `cni.cniBinDir` if using `helm template`
  cniBinDir: /opt/cni/bin