setup-gcp: no way to choose a cluster's release channel, so 1.37 is unreachable #1796

setup-gcp can pin a Kubernetes version (--cluster-version / CLUSTER_VERSION) but cannot choose a release channel, so every cluster it creates lands on whatever channel GKE defaults to (Regular today).

That is a problem as soon as the version you want is not carried by that channel. A channel only serves the versions it carries, and right now (checked against gcloud container get-server-config, us-central1) 1.37 is in Rapid alone:

So CLUSTER_VERSION=1.37 is simply rejected: setup-gcp cannot create a cluster on the release where the PodCertificate APIs are GA. Regular also defaults to 1.35.8, below what tools/setup-gcp/README.md documents as supported, which means a caller who passes no version at all gets a cluster the docs say is unsupported.

Proposal: add --release-channel / RELEASE_CHANNEL alongside the existing version flag on both bootstrap and create cluster, mapping to containerpb.ReleaseChannel.

Two details that matter:

• Empty and none are not the same. Leaving ReleaseChannel nil in the create request means "GKE decides", which is exactly what callers get today; sending an explicit UNSPECIFIED unenrolls the cluster from release channels. Conflating them would silently change behavior for everyone who passes nothing, so only a caller who asks for none should get UNSPECIFIED.
• extended can never work here. setup-gcp asks for the beta PodCertificate APIs on every cluster it creates, and the extended channel refuses beta APIs, so the create always fails with "enabling Beta APIs is not supported in the extended release channel". Worth rejecting before the API call rather than after.

Happy to send the PR — I have it written and tested.