Drupal\Core\Update\UpdateKernel::fixSerializedExtensionObjects() removed

Problem/Motivation

Follow-up to #3130341: Remove Drupal\Core\Update\UpdateKernel::fixSerializedExtensionObjects().

tests/src/Functional/Update360Test.php has:

 * This is a temporary workaround for issue #3031128; it can be removed when
 * UpdateKernel::fixSerializedExtensionObjects() is.
 */
protected function initConfig(ContainerInterface $container) {
  UpdateKernel::fixSerializedExtensionObjects($container);
  parent::initConfig($container);
}

Proposed resolution

Since Lightning Media 8.x-3.x is supported for 8.8.x, maybe it's worth wrapping it in a function_exists() check to maintain compatibility with both 8.8 and 9+? Unless 8.x-3.x will not support Drupal 9 anyway.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet