Istio overlays defined per environment and cluster
What
- Define Istio overlays per environment to deduplicate configs shared across all clusters.
- Fix typo on gstg-us-east1-c cluster name.
Why
- Related Issue: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/24157
- Related Component MR: gitlab-com/gl-infra/k8s-mgmt/components!135 (merged)