Null caches used while updating Drupal

Drupal uses the Update API to update data structures to match the expectations code. While this is happens data stored in the database cannot be trusted. Therefore the cache.factory is decorated by the \Drupal\Core\Update\UpdateServiceProvider to use \Drupal\Core\Update\UpdateBackend cache backends. These backends are null caches that pass on any cache deletes to the backends that are used when not updating. This means that we don't rely on any data stored in the database but any caches are emptied by the update process as expected.

As a result, the cache.null service added by \Drupal\Core\Update\UpdateServiceProvider is deprecated as it is no longer used.