Clean up references to SQL92

In most cases, these were just references to the SQL standard in
general.  In a few cases, a contrast was made between SQL92 and later
standards -- those have been kept unchanged.

src/test/regress/sql/case.sql

@@ -110,7 +110,7 @@ SELECT '' AS "Five",
 --
 -- NULLIF() and COALESCE()
 -- Shorthand forms for typical CASE constructs
---  defined in the SQL92 standard.
+--  defined in the SQL standard.
 --
 
 SELECT * FROM CASE_TBL WHERE COALESCE(f,i) = 4;