How to Hide Gutenberg Admin Menus

function plt_hide_gutenberg_menus() {
	//Hide "Appearance → Design".
	remove_submenu_page('themes.php', 'site-editor.php');

	//Hide "Gutenberg".
	remove_menu_page('gutenberg');
	//Hide "Gutenberg → Demo".
	remove_submenu_page('gutenberg', 'gutenberg');
	//Hide "Gutenberg → Support".
	remove_submenu_page('gutenberg', 'https://wordpress.org/support/plugin/gutenberg');
	//Hide "Gutenberg → Documentation".
	remove_submenu_page('gutenberg', 'https://developer.wordpress.org/block-editor');
	//Hide "Gutenberg → Experiments".
	remove_submenu_page('gutenberg', 'gutenberg-experiments');
}

add_action('admin_menu', 'plt_hide_gutenberg_menus', 11);
Where do I put this code?