Content translation metadata is now implemented through a translation wrapper

Content translation metadata is now accessed through an entity translation wrapper class. The Content Translation handler is responsible to expose field definitions for each metadata item. The wrapper provides methods to access the various metadata values. This approach allows one to provide an alternative set of field definitions for each entity type and to retrieve field values accordingly.

Before:

$metadata = $entity->translation; 
$source = $metadata[$langcode]['source'];
$outdated = $metadata[$langcode]['outdated'];

After:

$manager = \Drupal::service('content_translation.manager');
$metadata = $manager->getTranslationMetadata($entity->getTranslation($langcode));
$source = $metadata->getSource();
$outdated = $metadata->isOutdated();