Use MAX_PARALLEL_WORKER_LIMIT for max_parallel_maintenance_workers

max_parallel_maintenance_workers has been introduced in 9da0cc3528,
and used a hardcoded limit of 1024 rather than this variable.

max_parallel_workers and max_parallel_workers_per_gather already used
MAX_PARALLEL_WORKER_LIMIT (1024) as their upper-bound since
6599c9ac33.

Author: Matthias van de Meent
Reviewed-by: Zhang Mingli
Discussion: https://postgr.es/m/CAEze2WiCiJD+8Wig_wGPyn4vgdPjbnYXy2Rw+9KYi6izTMuP=w@mail.gmail.com

src/backend/utils/misc/guc_tables.c

@@ -3463,7 +3463,7 @@ struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&max_parallel_maintenance_workers,
-		2, 0, 1024,
+		2, 0, MAX_PARALLEL_WORKER_LIMIT,
 		NULL, NULL, NULL
 	},