Use doctrine for latest post fetching

[MAILPOET-2899]

lib/Newsletter/NewsletterPostsRepository.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace MailPoet\Newsletter;
+
+use MailPoet\Doctrine\Repository;
+use MailPoet\Entities\NewsletterPostEntity;
+
+/**
+ * @extends Repository<NewsletterPostEntity>
+ */
+class NewsletterPostsRepository extends Repository {
+  protected function getEntityClassName() {
+    return NewsletterPostEntity::class;
+  }
+}