Use MAX_PARALLEL_WORKER_LIMIT for max_parallel_maintenance_workers

Use MAX_PARALLEL_WORKER_LIMIT for max_parallel_maintenance_workers

max_parallel_maintenance_workers has been introduced in 9da0cc35284b,
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
6599c9ac3340.

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