An array of bundle keys, keyed by drupal core version.
In Drupal 6, only nodes were fieldable, and the bundles were called 'type_name'. In Drupal 7, everything became entities, and the more generic 'bundle' was used.
Type: array
protected $bundleKeys = [ FieldDiscoveryInterface::DRUPAL_6 => 'type_name', FieldDiscoveryInterface::DRUPAL_7 => 'bundle', ];