Migrate core menus into group menu bundles

Testing notes:

1. Enable the modules first:
1. ddev drush en sites_group -y
2. ddev drush en sites_migrator -y
3. ddev drush cr
2. Migrate the config
1. ddev drush sites-migrator:migrate-menu-config main
3. Enable Menüs for your site: /admin/group/types/manage/site/content
1. Note: here you might see an error "bundle null error". There might be a broken entity, because it has no bundle. That could be postponed. After you oben the site config anew, you'll see the enabled menu.
4. MIgrate the content
1. ddev drush sites-migrator:migrate-menu-content main 1 group_main "Test main navigation"
5. Now you'll see the migrated content in your new menu: /group/1/menu/1/edit

Maybe we should move the menu migration to an optional sub module. There might be potential use cases where someone doesn't need a menu migration or menu_item_extras...

But that can wait... The code looks good

Automatically closed - issue fixed for 2 weeks with no activity.