[FEATURE] ate-setup: remove GCP and Cloud SQL logic #3670

Removes the GCP and Cloud SQL behaviour from ate-setup. Provider installers pass explicit settings (kube context, token issuer, database connection) and supply provider configuration through Components.

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

Current state (0b635b80)

• gcp.* and Cloud SQL settings: cmd/ate-setup/internal/config/setting.go.
• gcloud credential fetch: config/credentials.go.
• JWT issuer derived from cluster coordinates: steps/create.go, config/setting.go.
• Google registry keychain: images/digest.go (also internal/e2e/upgrade/upgrade_test.go).
• PROJECT_ID, CLUSTER_NAME, CLUSTER_LOCATION exported to scripts: config/config.go.
• Cloud SQL proxy reconciliation: steps/cloudsql.go and manifests/ate-install/cloudsql/. It also removes a cloud-sql-proxy sidecar supplied by a Component.
• Snapshot transfer service account: SNAPSHOT_TRANSFER_GSA_NAME in config/setting.go.

Draft PR #3399 removes the Cloud SQL settings, but makes a set ATE_API_POSTGRES_CLOUDSQL_* an immediate hard error, with no warning release.

Required changes

• Remove everything listed above. Anything a GKE install still needs moves to the GKE Component or to explicit installer settings in substrate-gke (Installer passes explicit cluster settings to ate-setup ai-on-gke/substrate-gke#154, Installer takes over Cloud SQL adoption and configuration ai-on-gke/substrate-gke#155).
• Keep three safeguards:
  • refuse to switch an external-database install to bundled Postgres (exists, ate-setup: refuse to point an external-database install at the bundled one #2588);
  • fail when PROJECT_ID is set without --context, for two releases after removal;
  • accept removed keys with a warning for one release, and drop them when the install record is rewritten.

Dependencies

• [FEATURE] ate-setup: deprecation warnings for GCP and Cloud SQL settings #3669 shipped at least one release earlier.
• The GKE parity release (GKE parity release: install through the GKE Component ai-on-gke/substrate-gke#151).

Sequencing: ships in the same release as the other GKE default removals (#2589, #2760, #1258, #3668 step 2), so a GKE install never loses some defaults but not others.

Breaking change: yes. Release note with the replacement for each removed setting.

Enforcement. Deprecation tests (#3189): loading an old install record gives one warning per removed key, and the rewritten record omits them. A test asserts PROJECT_ID without --context fails.

Parent Issue: #3161