add missing return type hints

[ptmkenny]

Thanks for making it easy to create forks of modules!

Return type hints needs to be added for Symfony 7 compatibility. My CI builds (in PlatformSH) are failing with this error:

Return type declaration must be compatible with Command->execute(input: \Symfony\Component\Console\Input\InputInterface, output: \Symfony\Component\Console\Output\OutputInterface) : int

This PR adds the missing return type hints.