chore(deps): Update ghcr.io/cloudnative-pg/postgresql Docker tag to v16.15

platform/keycloak/postgres-cluster.yaml

@@ -41,7 +41,7 @@ spec:
   # not disconnected. (The anti-affinity change is a separate commit precisely because that one DOES
   # cause a switchover — see it for the blast radius.)
   instances: 3
-  imageName: ghcr.io/cloudnative-pg/postgresql:16.4
+  imageName: ghcr.io/cloudnative-pg/postgresql:16.15
   # HARD anti-affinity — the three instances MUST land on three DIFFERENT nodes (2026-09-07).
   #
   # CNPG's default is `preferred`, which is only a scheduler HINT: the scheduler may put two