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/ChillPersonBundle/Validator/Constraints/Person/Birthdate.php

@@ -20,9 +20,8 @@ use Symfony\Component\Validator\Constraint;
  * interval_spec : http://php.net/manual/en/dateinterval.construct.php
  * (this interval_spec itself is based on ISO8601 :
  *  https://en.wikipedia.org/wiki/ISO_8601#Durations)
- *
- * @Annotation
  */
+#[\Attribute(\Attribute::TARGET_PROPERTY)]
 class Birthdate extends Constraint
 {
     final public const BIRTHDATE_INVALID_CODE = '3f42fd96-0b2d-11ec-8cf3-0f3b1b1ca1c4';