pg_upgrade: Use COPY for LO metadata for upgrades from < v12.

pg_upgrade: Use COPY for LO metadata for upgrades from < v12.

Before v12, pg_largeobject_metadata was defined WITH OIDS, so
unlike newer versions, the "oid" column was a hidden system column
that pg_dump's getTableAttrs() will not pick up.  Thus, for commit
161a3e8b68, we did not bother trying to use COPY for
pg_largeobject_metadata for upgrades from older versions.  This
commit removes that restriction by adjusting the query in
getTableAttrs() to pick up the "oid" system column and by teaching
dumpTableData_copy() to use COPY (SELECT ...) for this catalog,
since system columns cannot be used in COPY's column list.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/aYzuAz_ITUpd9ZvH%40nathan