Hide concrete5 version number in upgrade page

If the constant `APP_VERSION_DISPLAY_IN_HEADER` (introduced in https://github.com/concrete5/concrete5/pull/723) is set to false, unregistered visitors can still determine the currently installed version by visiting the page `/index.php/tools/required/upgrade`.
Let's hide the version number when `APP_VERSION_DISPLAY_IN_HEADER` is set to false.