Replace any call to EntityType::isSubclassOf() with calls to EntityType::entityClassImplements()

EntityType::isSubclassOf() has been deprecated in Drupal 8.3.x and it will be removed in Drupal 10.x. The method still exists, but it triggers a deprecated error and it calls EntityType::entityClassImplements().

Any call to EntityType::isSubclassOf() needs to be replaced by a call to EntityType::entityClassImplements().

This change could be applied also to the 8.x-1.x branch, but since that branch can be installed with Drupal versions earlier than 8.3.x, I would limit this change only to the 9.1.x branch.