Draft: Resolve #3585018 "Drupal 12 support"

Closes #3585018.

This is a start.

I had to remove the dev dependency on entity_reference_revisions (which doesn't support D12) to get composer to build against Drupal 12. Of course, entity_reference_revisions is a requirement for some tests.

migrate_drupal is obsolete in D12 so related tests are now skipped against D12.

I think the best next step is to get entity_reference_revisions to support D12.

AI assisted: Claude identified solutions to test failures, which I have reviewed and accepted.