PostgreSQL backups run via the Barman Cloud Plugin

The Tuist CloudNativePG clusters now run backups through the Barman Cloud Plugin instead of the in-tree barmanObjectStore path, using a per-cluster ObjectStore CR with ScheduledBackup set to method: plugin. The archive bucket and serverName are unchanged, so WAL archiving continues against the same S3 prefix and any in-tree backups taken up to the cutover remain readable. Operators should also expect backup observability metrics to be renamed from cnpg_collector_* to barman_cloud_cloudnative_pg_io_*, and infra/cnpg/README.md now documents the plugin-based restore-drill procedure. The flag can be flipped back to false to resume the in-tree path during the transition.