add void to fix deprecation

[mattbild]

return void is needed in ExtensionInterface's implementation:

See deprecation message:
Method "Symfony\Component\DependencyInjection\Extension\ExtensionInterface::load()" might add "void" as a native return type declaration in the future.
Do the same in implementation "UnivLorraine\Bundle\SymfonyCasBundle\DependencyInjection\UnivLorraineSymfonyCasExtension" now to avoid errors or add an explicit @return annotation to suppress this message.

And replace Symfony\Component\Security\Core\Security:

Since symfony/security-bundle 6.2: The "Symfony\Component\Security\Core\Security" service alias is deprecated, use "Symfony\Bundle\SecurityBundle\Security" instead. It is being referenced by the "univ_lorraine_symfony_cas.authenticator" service.