fix(cnpg): switch shared-cluster to PG 18 with PostGIS + pgvector image

The Cluster CR was being rejected by the cnpg validating webhook:

  spec.postgresql.parameters.shared_preload_libraries: Invalid value: "vector":
    Can't set fixed configuration parameter

Two issues with that field: cnpg manages shared_preload_libraries itself
(it's a "fixed" parameter), and the user-facing field is at
spec.postgresql.shared_preload_libraries (top-level array), not under
parameters. pgvector doesn't actually require preloading — it loads on
CREATE EXTENSION — so dropping the line entirely.

Switch the operand image to ghcr.io/cloudnative-pg/postgis:18-3-system-trixie,
the cnpg-maintained image that bundles PostGIS, pgvector, pgaudit, and
barman-cloud (for backups). Bump the cnpg helm chart from v0.23.1
(operator 1.25.0) to v0.28.0 (operator 1.29.0) — required because PG 18
needs cnpg ≥ 1.26. Validated with `kubectl diff` against the current
1.25 webhook (exit 0, spec accepted).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: themightychris

.holo/sources/cloudnative-pg-chart.toml

@@ -1,3 +1,3 @@
 [holosource]
 url = "https://github.com/cloudnative-pg/charts.git"
-ref = "refs/tags/cloudnative-pg-v0.23.1"
+ref = "refs/tags/cloudnative-pg-v0.28.0"

infra/cloudnative-pg/shared-cluster.yaml

@@ -5,17 +5,13 @@ metadata:
   namespace: cloudnative-pg
 spec:
   instances: 3
-  imageName: ghcr.io/cloudnative-pg/postgresql:16.1
+  imageName: ghcr.io/cloudnative-pg/postgis:18-3-system-trixie
   storage:
     size: 20Gi
-  
+
   managed:
     roles:
       - name: balancer
         login: true
         passwordSecret:
           name: balancer-db-credentials
-
-  postgresql:
-    parameters:
-      shared_preload_libraries: 'vector'