Refactored code to use PHP8 attributes instead of annotations

In this change, Doctrine and validation annotations have been replaced with PHP8 Attributes. The Rector tool has been configured with a list of annotations to convert to attributes. As a consequence, the PHPStan's rules have been updated to reflect these changes. The PHP8's nullable operator (?) has been added where required, and comments in field declaration have been replaced with #[Attribute] syntax.

src/Bundle/ChillActivityBundle/Validator/Constraints/ActivityValidity.php

@@ -13,9 +13,7 @@ namespace Chill\ActivityBundle\Validator\Constraints;
 
 use Symfony\Component\Validator\Constraint;
 
-/**
- * @Annotation
- */
+#[\Attribute(\Attribute::TARGET_CLASS)]
 class ActivityValidity extends Constraint
 {
     final public const IS_REQUIRED_MESSAGE = ' is required';