Settings field needs to be updated to support getCustomSerializedPropertyNames method in Drupal 8.6.10

Since the release of Drupal 8.6.10 demo content import using the default_content module will fail with an error to the likes of:

The generic FieldItemNormalizer cannot denormalize string values for "settings" properties of the "example_field" field (field item class: Drupal\Core\Field\Plugin\Field\FieldType\StringLongItem).

They had the same issue in the paragraphs module: https://www.drupal.org/project/paragraphs/issues/3034598 →

I believe it was fixed with a single patch but it's not obvious to me how to fix it in the Formatter Field module.