pg_upgrade: Parallelize retrieving extension updates.

pg_upgrade: Parallelize retrieving extension updates.

This commit makes use of the new task framework in pg_upgrade to
parallelize retrieving the set of extensions that should be updated
with the ALTER EXTENSION command after upgrade.  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