Upgrade from Drupal 8 to Drupal 9

https://www.drupal.org/docs/upgrading-drupal/upgrading-from-drupal-8-to-drupal-9-or-later

https://dev.to/esnaremaussa/upgrading-from-drupal-8-to-drupal-9-with-composer-5639

https://www.drupal.org/docs/upgrading-drupal/how-to-prepare-your-drupal-7-or-8-site-for-drupal-9/upgrading-a-drupal-8-site

https://drupalize.me/tutorial/upgrade-drupal-9?p=2766

Bien penser à supprimer et remplacer drupal-composer/drupal-scaffold !

En cas de soucis d'upgrade selon l'historique du projet voir aussi https://www.drupal.org/docs/updating-drupal/migrating-the-composer-project-for-drupal-earlier-than-880