JSON API normalizers are removed from the serialization service.

JSON API's normalizers have been removed from the serializer service. Therefore, custom normalizers with a higher priority than JSON API's normalizers will no longer have any effect.

This change was necessary to indicate that the current JSON API serialization process is incompatible with the rest of Drupal's serialization system (see #2923779 for additional background).

The only level at which the JSON API module supports the standard normalization process is at the TypedData data type level. If you need to alter the serialization of attributes, this is the recommended approach.

Please remember that the JSON API normalization process is not a public API and there remains no backwards compatibility guarantee. However, a minority of existing applications may have overridden JSON APIs normalizers. Those projects may update their service tags from normalizer to jsonapi_normalizer_do_not_use_removal_imminent in order to stay up to date with latest feature and security releases while they remove their applications' dependencies on this internal API.