The instructions at https://ddev.readthedocs.io/en/latest/users/cli-usage/#drupal-9-quickstart clearly state how to set up a Drupal 9 project.
However, what is the procedure to upgrade an up to date Drupal 8 project to Drupal 9 in DDEV?
We're assuming the "project-type" needs changing, perhaps PHP and DB versions, etc.