Trigger an error on direct access of ContentEntityStorageBase::doLoadMultipleRevisionFieldItems() and mark it to be set abstract in Drupal 9

So, this is fun.

  /**
   * Actually loads revision field item values from the storage.
   *
   * @param int|string $revision_id
   *   The revision identifier.
   *
   * @return \Drupal\Core\Entity\EntityInterface|null
   *   The specified entity revision or NULL if not found.
   *
   * @deprecated in Drupal 8.5.x and will be removed before Drupal 9.0.0.
   *   \Drupal\Core\Entity\ContentEntityStorageBase::doLoadMultipleRevisionsFieldItems()
   *   should be implemented instead.
   *
   * @see https://www.drupal.org/node/2924915
   */
  abstract protected function doLoadRevisionFieldItems($revision_id);

  /**
   * Actually loads revision field item values from the storage.
   *
   * @param array $revision_ids
   *   An array of revision identifiers.
   *
   * @return \Drupal\Core\Entity\EntityInterface[]
   *   The specified entity revisions or an empty array if none are found.
   */
  protected function doLoadMultipleRevisionsFieldItems($revision_ids) {
    $revisions = [];
    foreach ($revision_ids as $revision_id) {
      $revisions[] = $this->doLoadRevisionFieldItems($revision_id);
    }

    return $revisions;
  }

I think ContentEntityBase::doLoadMultipleRevisionFieldItems() needs to be abstract, and the extending storage classes need to add their loading logic directly. So we should trigger an error from here, and make it deprecated for inheriting classes to call parent::doLoadMultipleRevisionsFieldItems(), (core doesn't) and then make the method abstract in D9

Oh, those were two different links.. the @see was the CR explaining that calling the method directly was deprecated, the @todo linked to the followup issue.

The CR is still linked in the @trigger_error though, so it should be fine.