How to Enable Maintenance Mode on Your WordPress Site for All Users Except Administrators

<?php

add_action('template_redirect', function() {
    if( !current_user_can( 'manage_options') && !is_admin()) {
        status_header(503);
        ?>
        <h1 style="font-family:system-ui;text-align:center;margin-top:100px;">
        The site is currently in maintenance mode. Check back soon!
        </h1>
        <?php
        exit;
    }
});