Use MAX_PARALLEL_WORKER_LIMIT consistently in guc_tables.c

Hi,

Whilst doing some digging in parallel code, I noticed that
max_parallel_maintenance_workers is registered as guc with a manual
value of 1024, while max_parallel_workers_per_gather uses
MAX_PARALLEL_WORKER_LIMIT (also 1024). After some archeology, the
discrepancy seems to have existed ever since
max_parallel_maintenance_workers was originally introduced, as the
patch development that introduced the GUC that eventually got
committed predates the use of MAX_PARALLEL_WORKER_LIMIT in guc.c (now
guc_tables.c), and the change to the definition of sibling GUCs of
max_parallel_workers and max_parallel_workers_per_gather wasn't
noticed during that development.

PFA a trivial one-line patch that makes that a bit more consistent.

Kind regards,

Matthias van de Meent