[9.20] chg: dev: Restore the number of threadpool threads back to original value

The issue of long-running operations potentially blocking query resolution has been fixed. Revert this temporary workaround and restore the number of threadpool threads.

Related #4898

Backport of MR !9530

Merge branch 'backport-4898-remove-workaround-and-note-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9532