fix(helm): revert copilot-postgresql STS serviceName too (same immutability issue)

Audit caught that the main fix in d5c2e8e missed statefulset-copilot-postgres.yaml,
which had the identical immutable-field rename from -copilot-postgresql to
-copilot-postgresql-headless. Same upgrade-break vector for anyone running
copilot.enabled=true on a prior chart version. Mirrors the fix and comment
from the main postgresql STS.

Author: waleedlatif1

helm/sim/templates/statefulset-copilot-postgres.yaml

@@ -66,7 +66,11 @@ metadata:
     {{- include "sim.labels" . | nindent 4 }}
     app.kubernetes.io/component: copilot-postgresql
 spec:
-  serviceName: {{ include "sim.fullname" . }}-copilot-postgresql-headless
+  # Must remain {{ include "sim.fullname" . }}-copilot-postgresql (not the
+  # -headless name) — spec.serviceName is immutable on a StatefulSet, and
+  # the prior chart shipped with this value. Same rationale as the main
+  # postgresql STS; see statefulset-postgresql.yaml for details.
+  serviceName: {{ include "sim.fullname" . }}-copilot-postgresql
   replicas: 1
   podManagementPolicy: OrderedReady
   updateStrategy: