- config.load_defaults 6.0 in the dummy app and
  fix the test since by default rails 6.0 configured
  does not generate "utf8" hidden input, see #32125
- Use `ActiveRecord::Migration[6.0]` in the dummy app
  since actiontext will be since Rails 6.0
- Fix `CreateActiveStorageTables` migration in the dummy app.
  Add `t.foreign_key :active_storage_blobs, column: :blob_id`
  It was added in 2ae3a295.
- `rails/actiontext$ yarn install`