Special characters on config for localgov_menu_link_group

Problem/Motivation

Configuration for localgov_menu_link_group fails due to special characters.

Steps to reproduce

1. Install localgov_drupal
2. Enable localgov_france
3. Export the config drush cex -y
4. Edit config/sync/localgov_menu_link_group.localgov_menu_link_group.localgov_menu_link_group_news.yml and replace "Actualités" by "Actualites"
5. Execute
   

   drush cim</li>
     <li>Execute <code>drush cr

6. Edit again config/sync/localgov_menu_link_group.localgov_menu_link_group.localgov_menu_link_group_news.yml and replace "Actualites" by "Actualités"
7. Execute
   

   drush cim</li>
     <li>Execute <code>drush cr

Proposed resolution

`prepareMenuLinkIndexForGroup` in MenuLinkGrouper is trying to generate a machine name based on the label, however the label may contains special characters like "Actualités". This is then failing while importing the config.

The proposal would be to make use of the transliteration service to prevent this to happen and use it over `$group->label()`

Remaining tasks

1. Create a MR with the fix