PG-1313 Fix decode_error_level SQL function (#539)

Update decode_error_level funcion to support error codes up to PG
version 17.

regression/sql/decode_error_level.sql

@@ -0,0 +1,12 @@
+CREATE EXTENSION pg_stat_monitor;
+
+DO $$
+DECLARE
+    i integer;
+BEGIN
+    FOR i IN 10..24 LOOP
+         RAISE NOTICE 'error_code: %, error_level: %', i, decode_error_level(i);
+    END LOOP;
+END $$;
+
+DROP EXTENSION pg_stat_monitor;