Add support for Symfony 6.x

[ancarda]

This commit patches AddToCartListener to resolve an error caused by running this plugin on Symfony 6.x:

[Symfony\Component\HttpKernel\Exception\BadRequestHttpException]
Input value "sylius_add_to_cart" contains a non-scalar value

See: https://github.com/symfony/symfony/blob/6.0/UPGRADE-6.0.md#httpfoundation

Retrieving non-scalar values using InputBag::get() will throw BadRequestException (use InputBag::all() instead to retrieve an array)

Additionally, this commit drops support for Symfony 4.x, as Sylius itself no longer supports 4.x.

[mamazu]

Thanks for the help.