Replace deprecated UnitOfWork::clear($entityClassName) with a more granular refresh

We removed EntityManager::clear($entityClassName) already, but this one is deprecated
as well: 212edaa80b/lib/Doctrine/ORM/UnitOfWork.php (L2762)

[MAILPOET-5745]

mailpoet/lib/Settings/SettingsRepository.php

@@ -6,13 +6,22 @@ use MailPoet\Doctrine\Repository;
 use MailPoet\Entities\SettingEntity;
 use MailPoet\WP\Functions as WPFunctions;
 use MailPoetVendor\Carbon\Carbon;
+use MailPoetVendor\Doctrine\ORM\Query;
 
 /**
  * @extends Repository<SettingEntity>
  */
 class SettingsRepository extends Repository {
-  public function findOneByName($name) {
-    return $this->doctrineRepository->findOneBy(['name' => $name]);
+  public function findOneByName(string $name): ?SettingEntity {
+    // Always fetch fresh entity data (= don't use "findOneBy()"). See also further below.
+    $result = (array)$this->doctrineRepository->createQueryBuilder('s')
+      ->where('s.name = :name')
+      ->setParameter('name', $name)
+      ->getQuery()
+      ->setHint(Query::HINT_REFRESH, true)
+      ->getResult();
+
+    return isset($result[0]) && $result[0] instanceof SettingEntity ? $result[0] : null;
   }
 
   public function createOrUpdateByName($name, $value) {
@@ -30,7 +39,11 @@ class SettingsRepository extends Repository {
       'value' => is_array($value) ? serialize($value) : $value,
       'now' => $now,
     ]);
-    $this->entityManager->getUnitOfWork()->clear(SettingEntity::class);
+
+    // Ensure entity data is up-to-date in memory.
+    //  - We can't use "refresh()"; we don't have the entity instance, and it can be a new record.
+    //  - We can't use "findOneBy()"; it could return a cached entity instance.
+    $this->findOneByName($name);
   }
 
   protected function getEntityClassName() {