Cosmetic improvement: use BKI_DEFAULT and BKI_LOOKUP in pg_language.

Cosmetic improvement: use BKI_DEFAULT and BKI_LOOKUP in pg_language.

The point of this is not really to remove redundancy in pg_language.dat;
with only three entries, it's hardly worth it.  Rather, it is to get
to a point where there are exactly zero hard-coded numeric pg_proc OID
references in the catalog .dat files.  The lanvalidator column was the
only remaining location of such references, and it seems like a good
thing for future-proofing reasons to make it not be a special case.

There are still a few places in the .dat files with numeric OID references
to other catalogs, but after review I don't see any that seem worth
changing at present.  In each case there are just too few entries to make
it worth the trouble to create lookup infrastructure.

This doesn't change the emitted postgres.bki file, so no catversion bump.