Switch to correct Symfony Extension namespace

[Lustmored]

This PR fixes the deprecations:

The "Symfony\Component\HttpKernel\DependencyInjection\Extension" class is considered internal since Symfony 7.1, to be deprecated in 8.1; use Symfony\Component\DependencyInjection\Extension\Extension instead. It may change without further notice. You should not use it from "Ddeboer\VatinBundle\DependencyInjection\DdeboerVatinExtension".

The correct extension class is available in all Symfony versions supported by this bundle and this change has no side effects.

[ddeboer]

Thanks!