Support for Symfony 8.1

[stevenbrookes]

Seeing the following deprecations warning

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