Issue #2969806 by mihai11, Jo Fitzgerald: Fix "CckFile is deprecated in Drupal...

Issue #2969806 by mihai11, Jo Fitzgerald: Fix "CckFile is deprecated in Drupal 8.3.x and will be be removed before Drupal 9.0.x. Use \Drupal\file\Plugin\migrate\process\d6\FieldFile instead." deprecation message