\Drupal\Core\Plugin\Mapper\MapperInterface is no longer required for plugin managers

\Drupal\Core\Plugin\Mapper\PluginManagerInterface no longer extends Drupal\Component\Plugin\Mapper\MapperInterface. This means plugin managers no longer have to implement MapperInterface::getInstance(). In addition, \Drupal\Component\Plugin\PluginManagerBase and therefore \Drupal\Core\Plugin\DefaultPluginManager no longer implement MapperInterface.
Before:

interface PluginManagerInterface extends DiscoveryInterface, FactoryInterface, MapperInterface {
}

After:

interface PluginManagerInterface extends DiscoveryInterface, FactoryInterface {
}