pg_upgrade: Parallelize postfix operator check.
This commit makes use of the new task framework in pg_upgrade to
parallelize the check for user-defined postfix operators. This
step will now process multiple databases concurrently when
pg_upgrade's --jobs option is provided a value greater than 1.
Reviewed-by: Daniel Gustafsson, Ilya Gladyshev
Discussion: https://postgr.es/m/
20240516211638.GA1688936%40nathanxps13