Fix GCP setup broken link in dev docs

[NavidZ]

Changes

The current link to the GKE setup in Knative repo is broken.
They removed the specific k8s setups and now have a generic
one here:
https://github.com/knative/docs/blob/master/docs/install/any-kubernetes-cluster.md
which doesn't talk about GCP at all.

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[tekton-robot]

Hi @NavidZ. Thanks for your PR.

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[vdemeester]

/ok-to-test
/cc @bobcatfish @sbwsg @dibyom
/lgtm

[tekton-robot]

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