fix: (IAC-1008) Explicitly Set the release_channel when Creating GKE Clusters

[jarpat]

Changes

When the user sets UNSPECIFIED for the kubernetes_channel do not set release_channel to null and rely on the default value downstream. Instead we will be explicitly setting release_channel it to the value in kubernetes_channel.

We are doing this because apparently there was a underlying GKE API change where setting the release_channel to null would now default to the REGULAR channel. It was advised to no longer set this value to null. See this GitHub issue I opened against the terraform-google-modules/kubernetes-engine module
terraform-google-modules/terraform-google-kubernetes-engine#1620

They are working on getting the doc updated in the next module release.

Note:

• kubernetes_channel: the variable we expose to users in this project to control the release channel
• release_channel: the parameter that controls the release channel in the terraform-google-modules/kubernetes-engine/google//modules/private-cluster module

Tests

Scenario	Provider	kubernetes_version	kubernetes_channel	Actual K8s Version
1	GCP	1.25	unset (default UNSPECIFIED)	v1.25.8-gke.1000
2	GCP	1.24.10	UNSPECIFIED	v1.24.10-gke.2300
3	GCP	unset	REGULAR	1.25.7-gke.1000