chore: fix deprecations in preparation uf upgrading to symfony 7.3

src/Form/Model/ParticipantDto.php

@@ -40,6 +40,7 @@ class ParticipantDto
     public ?int $personId = null;
     public ?string $status = null;
     public bool $mutable = false;
+    public bool $touched = false;
 
     #[Assert\NotBlank(message: 'Bitte angeben', groups: ['booking_edit', 'booking_create'])]
     public ?string $firstName = null;