Proper annotation for generated validator constraint

[zorn-v]

Currently, if you are using phpstan in your project you'll get

Access to an undefined property Symfony\Component\Validator\Constraint::$message

error on any new generated validator

[drieschel]

I tested it and it works. 👍

Do you think it makes sense also to add the type to the $message property in src/Resources/skeleton/validator/Constraint.tpl.php in this mr?

public string $message = 'The value "{{ value }}" is not valid.';

[weaverryan]

Thanks @zorn-v!