How to get the field settings values in filed constraint validator
If they are field storage settings, then you could pass them along as arguments to the constraint.

If not, then add the constraint on the field type, not on the property. You can do that directly in the annotation, see \Drupal\file\Plugin\Field\FieldType\FileItem as an exampe. Then you get the whole field item in your validator and you can access the values and the definition/settings.