Fix constant when extracting timestamp from UUIDv7.

When extracting a timestamp from a UUIDv7, a conversion from
milliseconds to microseconds was using the incorrect constant
NS_PER_US instead of US_PER_MS. Although both constants have the same
value, this fix improves code clarity by using the semantically
correct constant.

Backpatch to v18, where UUIDv7 was introduced.

Author: Erik Nordström <erik@tigerdata.com>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Discussion: https://postgr.es/m/CACAa4V+i07eaP6h4MHNydZeX47kkLPwAg0sqe67R=M5tLdxNuQ@mail.gmail.com
Backpatch-through: 18

src/backend/utils/adt/uuid.c

@@ -752,7 +752,7 @@ uuid_extract_timestamp(PG_FUNCTION_ARGS)
 			+ (((uint64) uuid->data[0]) << 40);
 
 		/* convert ms to us, then adjust */
-		ts = (TimestampTz) (tms * NS_PER_US) -
+		ts = (TimestampTz) (tms * US_PER_MS) -
 			(POSTGRES_EPOCH_JDATE - UNIX_EPOCH_JDATE) * SECS_PER_DAY * USECS_PER_SEC;
 
 		PG_RETURN_TIMESTAMPTZ(ts);