[Rector] Applied all Symfony 5.x rectors to the production codebase

Applied rules:
 * AddReturnTypeDeclarationBasedOnParentClassMethodRector
 * AddReturnTypeDeclarationRector

src/lib/ContentView/QueryResultsInjector.php

@@ -40,7 +40,7 @@ public function __construct(QueryFieldServiceInterface $queryFieldService, array
         $this->requestStack = $requestStack;
     }
 
-    public static function getSubscribedEvents()
+    public static function getSubscribedEvents(): array
     {
         return [ViewEvents::FILTER_VIEW_PARAMETERS => 'injectQueryResults'];
     }

src/lib/FieldType/Form/QueryFieldFormType.php

@@ -28,12 +28,12 @@ public function getName()
         return $this->getBlockPrefix();
     }
 
-    public function getBlockPrefix()
+    public function getBlockPrefix(): string
     {
         return 'ezplatform_fieldtype_query';
     }
 
-    public function getParent()
+    public function getParent(): ?string
     {
         return TextType::class;
     }

src/lib/FieldType/Query/Value.php

@@ -31,7 +31,7 @@ public function __construct($text = '')
     /**
      * @see \Ibexa\Core\FieldType\Value
      */
-    public function __toString()
+    public function __toString(): string
     {
         return (string)$this->text;
     }