Fix new pg_upgrade query not to rely on regnamespace

That was invented in 9.5, and pg_upgrade claims to support back to 9.0.
But we don't need that with a simple query change, tested by Tom Lane.

Discussion: https://postgr.es/m/202507041645.afjl5rssvrgu@alvherre.pgsql

src/bin/pg_upgrade/check.c

@@ -1239,12 +1239,13 @@ check_for_not_null_inheritance(ClusterInfo *cluster)
 		PGconn	   *conn = connectToServer(&old_cluster, active_db->db_name);
 
 		res = executeQueryOrDie(conn,
-								"SELECT cc.relnamespace::pg_catalog.regnamespace AS nspname, "
-								"       cc.relname, ac.attname "
+								"SELECT nspname, cc.relname, ac.attname "
 								"FROM pg_catalog.pg_inherits i, pg_catalog.pg_attribute ac, "
-								"     pg_catalog.pg_attribute ap, pg_catalog.pg_class cc "
+								"     pg_catalog.pg_attribute ap, pg_catalog.pg_class cc, "
+								"     pg_catalog.pg_namespace nc "
 								"WHERE cc.oid = ac.attrelid AND i.inhrelid = ac.attrelid "
 								"      AND i.inhparent = ap.attrelid AND ac.attname = ap.attname "
+								"      AND cc.relnamespace = nc.oid "
 								"      AND ap.attnum > 0 and ap.attnotnull AND NOT ac.attnotnull");
 
 		ntup = PQntuples(res);