\Drupal\jsonapi\JsonApiResource\Link::merge now throws an assertion error when links with different relation types or target attributes are merged

In the previous release, it was possible to merge two links that shared a common URI but differing link relation types or target attributes. Doing so could lead Drupal\jsonapi\JsonApiResource\Link::merge to return a nonsensical merged link. To preclude this from happening, Link::merge will now raise an assertion error instead. Callers should have already been Drupal\jsonapi\JsonApiResource\Link::compare before attempting to merge links. Those callers that are doing so will not need to make any updates because Link::compare no longer treats links with different link relation types or target attributes as equivalent.