fix: Add serverName to CNPG externalClusters for B2 recovery

CNPG uses the external cluster name (b2-backup) as the barman server
name by default, but the backups were stored under server name rancher-db.

infrastructure/addons/cnpg/postgres-cluster.yaml

@@ -42,6 +42,7 @@ spec:
       barmanObjectStore:
         destinationPath: "s3://HetznerTerra/rancher-backups/"
         endpointURL: "https://s3.us-east-005.backblazeb2.com"
+        serverName: rancher-db
         s3Credentials:
           accessKeyId:
             name: b2-credentials