MBS-10347: Include modal option in backup

Author: sh-csg

backup/moodle2/backup_learningmap_stepslib.php

@@ -43,6 +43,7 @@ protected function define_structure(): backup_nested_element {
                 'backlink',
                 'svgcode',
                 'showmaponcoursepage',
+                'usemodal',
             ]
         );