chore: make GKE cluster VPC-native

[bschaatsbergen]

• Changed the existing GKE cluster to be a VPC-native cluster
• This includes a vpc.tf to provision a custom VPC using Google Private Access
• Added a couple firewall rules to the VPC in order to allow IAP TCP forwarding
• The GKE cluster is public/private (public control-plane with private-nodes), making it easier for the practitioner to connect to it using kubectl or through the Kubernetes/Helm Terraform Providers.

Possible breaking change in the workflow: I've removed the setup/ dir, as this was for TFC only? @ksatirli - we can put it back if the intention is to deploy through TFC.