Update Helm release postgres-cluster to v6.15.0 (#1863)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [postgres-cluster](https://github.com/cloudnative-pg/charts) ([source](https://github.com/cloudnative-pg/charts/tree/HEAD/charts/cluster)) | minor | `6.14.0` -> `6.15.0` |

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Reviewed-on: #1863
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Co-committed-by: Renovate Bot <renovate-bot@alexlebens.net>

clusters/cl01tl/applications/roundcube/Chart.yaml

@@ -21,7 +21,7 @@ dependencies:
     version: 4.4.0
   - name: postgres-cluster
     alias: postgres-17-cluster
-    version: 6.14.0
+    version: 6.15.0
     repository: oci://harbor.alexlebens.net/helm-charts
 icon: https://cdn.jsdelivr.net/gh/selfhst/icons/png/roundcube.png
 appVersion: 1.6.10