Fix a problem in the codebase where we create, schedule and update newsletters in the user time zone, but we set the ProcessedAt with another timezone
Also, `Carbon::createFromTimestamp($this->wp->currentTime('mysql'))` is not a valid date.
It would return date: '1970-01-01 00:35:29.000000'
MAILPOET-5795
This commit updates the method ScheduledTaskSubscribersRepository::checkCompleted() to update the processedAt field of the Scheduled Task. The method was added in f62d9b4167 and I believe it is meant to be a replacement to \MailPoet\Tasks\Subscribers::checkCompleted(). The original method updates the processedAt field, so I'm assuming the new method should update as well. I found this while working on refactoring \MailPoet\Tasks\Sending::removeSubscribers() to use Doctrine as this method calls \MailPoet\Tasks\Subscribers::checkCompleted() internaly.
[MAILPOET-4368]
