cephrgw: allow purge-on-delete for cnpg backup buckets (ec migration step 1)

Claude-Session: https://claude.ai/code/session_015ur3i7D2azsMAWTSVABApv

apps/base/artifactapi/cnpg_backup.yaml

@@ -36,7 +36,8 @@ spec:
     app: artifactapi
     purpose: cnpg-backup
   # Never drop the backups if the CR is removed; retire buckets by hand.
-  retainOnDelete: true
+  retainOnDelete: false
+  purgeOnDelete: true
 ---
 # Nightly base backup. Continuous WAL archiving is always-on via the Cluster's
 # spec.backup.barmanObjectStore; this schedules the periodic full backup that