Followup on https://github.com/rails/rails/pull/41258#discussion_r570441592



This makes sure that newly generated applications get their
`ApplicationRecord` set to `primary_abstract_class`. Existing applications
can opt-in to this if they're using multiple databases or have changed
their `ApplicationRecord` to a class with a different name.

Co-authored-by: John Crepezzi <john.crepezzi@gmail.com>