Add Different Class on Different Ul in twenty-twentyone theme

add_li_class isn’t valid arg for wp_nav_menu

Try this in your functions.php

add_filter( 'nav_menu_css_class', 'add_my_class_to_nav_menu', 10, 2 );
function add_my_class_to_nav_menu( $classes, $item ){
    $classes[] = 'menu-dropdown';
    return $classes;
}