Update check_requirements to allow MU installs

Hello thank you for the plugin! However, we are having some issues installing it using YOAST as a MU plugin. I have modified the requirement check as follows to make it work, but it would be great to have this in an official release! Thank you!

    public function check_requirements() {
        if ( ! defined( 'WPSEO_VERSION' ) ) {
            deactivate_plugins( 'wp-rest-yoast-meta/wp-rest-yoast-meta.php' );
            add_action( 'admin_notices', [ $this, 'requirements_notice' ] );

            if ( isset( $_GET['activate'] ) ) { // WPCS: CSRF ok.
                unset( $_GET['activate'] );
            }
        }
    }