Update SQL conformance information

T612 has been fully supported since the major window function
enhancements in PostgreSQL 11, but it wasn't updated at the time.

src/backend/catalog/sql_features.txt

@@ -508,7 +508,7 @@ T581	Regular expression substring function			YES
 T591	UNIQUE constraints of possibly null columns			YES	
 T601	Local cursor references			NO	
 T611	Elementary OLAP operations			YES	
-T612	Advanced OLAP operations			NO	some forms supported
+T612	Advanced OLAP operations			YES	
 T613	Sampling			YES	
 T614	NTILE function			YES	
 T615	LEAD and LAG functions			YES