Update Helm release rook-ceph-cluster to v1.18.6

Author: renovate[bot]

fleet/lib/rook-ceph-cluster/fleet.yaml

@@ -8,7 +8,7 @@ helm:
   chart: *name
   releaseName: *name
   repo: https://charts.rook.io/release
-  version: v1.18.4
+  version: v1.18.6
   valuesFiles:
     - values.yaml
   timeoutSeconds: 3600