How to get node id when update a page related to menu link

In my module I use the mymodulename_entity_update() function to listen when an entity is updated:

function mymodulename_entity_update($entity) {
    file_put_contents(__DIR__.'/test.txt', $entity->bundle());
}

When I update an article or a page which is not related to menu link, I receive 'node' as value of the ->bundle() property, as expected.

But if I update a page which is related to a menu link, I receive the 'menu_link_content' as the bundle value and all the data of the $entity object is the data of menu link. The same data I receive when update a menu link. And the only thing that helps me to detect whether a page was updated or menu link is the ->expanded->value property: in the case of a page it equals 0.

The second problem is, how to get ID of the node which is related to the menu link?

I was trying $entity->getRouteParameters()['node'] as recommended by the link https://drupal.stackexchange.com/a/281646/104765 but I receive the error

Call to undefined method Drupal\menu_link_content\Entity\MenuLinkContent::getRouteParameters()...