I am working on implementing the Group module on Drupal 9. The Release Notes describe moving from Group 2.0.x to 3.0.x as a 'Migration', which I take it is not just an upgrade that can be done using Composer.
Our overall project plan means we will need to do this migration on a site containing Group data, with associated content (Nodes) and members (Users).
Without considering the migration of custom modules, how can we perform this migration?