Use canonical DateStyle name (#2925)

This makes caching variable caching with PgBouncer more efficient, because now the client is setting the same values that the server reports.

See these PgBouncer issues/PRs for details:
pgbouncer/pgbouncer#776
pgbouncer/pgbouncer#879

pgjdbc/src/main/java/org/postgresql/core/v3/ConnectionFactoryImpl.java

@@ -375,7 +375,7 @@ private List<StartupParam> getParametersForStartup(String user, String database,
     paramList.add(new StartupParam("user", user));
     paramList.add(new StartupParam("database", database));
     paramList.add(new StartupParam("client_encoding", "UTF8"));
-    paramList.add(new StartupParam("DateStyle", "ISO"));
+    paramList.add(new StartupParam("DateStyle", "ISO, MDY"));
     paramList.add(new StartupParam("TimeZone", createPostgresTimeZone()));
 
     Version assumeVersion = ServerVersion.from(PGProperty.ASSUME_MIN_SERVER_VERSION.getOrDefault(info));