MigrationEvent improvements

[HermanPeeren]

• Why do you override the constructor and the getArguments() method? Doesn't make sense to me. Look at the code of the AbstractEvent parent.
• You use a property $results (an array), but you didn't declare that property, nor is it a property of an ancester of MigrationEvent. Hence: you created the property dynamically. But: In PHP 8.2, Dynamic Properties are deprecated (https://wiki.php.net/rfc/deprecate_dynamic_properties), and will result in a fatal error from PHP 9 onwards.
• Results in Joomla's Events are stored in the $arguments['result'] array. You should implement the Joomla\CMS\Event\Result\ResultAwareInterface in your Event and use the Joomla\CMS\Event\Result\ResultAware trait, so the 'result' is added as a key to the $arguments array. You'll have to implement the getResults() method yourself, it is not in the ResultAware trait. That method should retrieve $this->getArgument('result', []). You use that getResults()method in your ImportModel, line 69.