[FEATURE] ate-setup: deprecation warnings for GCP and Cloud SQL settings #3669

Warns one release ahead that the GCP and Cloud SQL behaviour in ate-setup is being removed, so operators and provider installers can switch to explicit settings before it breaks.

Tracks "Remove GCP and Cloud SQL logic from ate-setup", warning release, in the Provider-Neutral Core workstream of the Extensible Install Roadmap.

Current state (0b635b80): no deprecation warnings in cmd/ate-setup/internal/config/.

Required changes. Print one warning per key, naming its replacement or substrate-gke, when any of these is used:

• --gcp-project-id, --gcp-cluster-name, --gcp-cluster-location (PROJECT_ID, CLUSTER_NAME, CLUSTER_LOCATION);
• --ateapi-postgres-cloudsql-* (ATE_API_POSTGRES_CLOUDSQL_*);
• implicit behaviour: fetching credentials with gcloud container clusters get-credentials, deriving the JWT issuer from cluster coordinates, the Google registry keychain fallback, and the snapshot transfer service account setting (SNAPSHOT_TRANSFER_GSA_NAME).

Behaviour is otherwise unchanged.

Dependencies: none.

Sequencing: ships at least one release before the removal in #3670.

Breaking change: none.

Enforcement: CLI tests assert exactly one warning per used key, and none for an install that sets no GCP or Cloud SQL setting.

Parent Issue: #3161