How to Hide the "Astra Starter Sites" Admin Menu

function plt_hide_astra_sites_menus() {
	//Hide the "Appearance → Astra Starter Sites" menu.
	remove_submenu_page('themes.php', 'astra-sites');
}

add_action('admin_menu', 'plt_hide_astra_sites_menus', 101);

Where do I put this code?