Add compatibility with symfony/validator v6

Anatoliy Melnikovrequested to merge
aeliot-tm/graphql-mutation-validator-bundle:add_compatibility_with_validator_v6 into master

Backward compatibility break There is declared mixed data type of the first parameter of the method \AssoConnect\GraphQLMutationValidatorBundle\Validator\Constraints\GraphQLRequestObjectValidator::validate(). That is required by the interface \Symfony\Component\Validator\ConstraintValidatorInterface. And this is locked to PHP ^8.0

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