Clears available update status data.
Since this function is run at such a low bootstrap level, the Update Manager module is not loaded. So, we can't just call update_storage_clear(). However, the key-value backend is available, so we just call that.
Note that we do not want to delete items related to currently pending fetch attempts.
update_authorize_update_batch_finished()
function _update_authorize_clear_update_status() {
Drupal::keyValueExpirable('update')
->deleteAll();
Drupal::keyValueExpirable('update_available_release')
->deleteAll();
}