Codechange: Use EnumBitSet for WindowDefaultFlags.

src/intro_gui.cpp

@@ -459,7 +459,7 @@ static constexpr NWidgetPart _nested_select_game_widgets[] = {
 static WindowDesc _select_game_desc(
 	WDP_CENTER, nullptr, 0, 0,
 	WC_SELECT_GAME, WC_NONE,
-	WDF_NO_CLOSE,
+	WindowDefaultFlag::NoClose,
 	_nested_select_game_widgets
 );