Fix checkGenericClassInNonGenericObjectType PHPStan errors

This commit removes the checkGenericClassInNonGenericObjectType flag
from PHPStan configuration files and fixes all associated errors in our
code base.

[MAILPOET-3236]

lib/Entities/SubscriberEntity.php

@@ -139,7 +139,7 @@ class SubscriberEntity {
 
   /**
    * @ORM\OneToMany(targetEntity="MailPoet\Entities\SubscriberSegmentEntity", mappedBy="subscriber")
-   * @var iterable<SubscriberSegmentEntity>&Collection
+   * @var Collection<int, SubscriberSegmentEntity>
    */
   private $subscriberSegments;
 
@@ -395,7 +395,7 @@ class SubscriberEntity {
   }
 
   /**
-   * @return Collection
+   * @return Collection<int, SubscriberSegmentEntity>
    */
   public function getSubscriberSegments() {
     return $this->subscriberSegments;