Backup Redis Cluster. If this is the first time a backup is being created, a backup collection will be created at the backend, and this backup belongs to this collection. Both collection and backup will have a resource name. Backup will be executed for each shard. A replica (primary if nonHA) will be selected to perform the execution. Backup call will be rejected if there is an ongoing backup or update operation. Be aware that during preview, if the cluster's internal software version is too old, critical update will be performed before actual backup. Once the internal software version is updated to the minimum version required by the backup feature, subsequent backups will not require critical update. After preview, there will be no critical update needed for backup.