pg_upgrade: Parallelize contrib/isn check.
This commit makes use of the new task framework in pg_upgrade to
parallelize the check for contrib/isn functions that rely on the
bigint data type. 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