Refactor MetaInfo::getConfirmationMetaInfo() to use Doctrine

This method is used only inside
ConfirmationEmailMailer::sendConfirmationEmail(). So I figured it was
best to refactor it as well to use SubscriberEntity instead of the old
Subscriber model as part of the task to do the same for the
ConfirmationEmailMailer class. I opted for this approach since it was a
straightforward change.

[MAILPOET-3815]

lib/Subscribers/ConfirmationEmailMailer.php

@@ -121,7 +121,7 @@ class ConfirmationEmailMailer {
     // send email
     try {
       $extraParams = [
-        'meta' => $this->mailerMetaInfo->getConfirmationMetaInfo($subscriber),
+        'meta' => $this->mailerMetaInfo->getConfirmationMetaInfo($subscriberEntity),
       ];
       $result = $this->mailer->send($email, $subscriber, $extraParams);
       if ($result['response'] === false) {