Adopt gitlab-styles for our formatting

Right now, we have our own rubocop.yml and we use rufo for formatting.

Though I like the idea of rufo making decisions and being unobtrusive, I don't like that a minor version bump can break the build (https://gitlab.com/gitlab-org/labkit-ruby/issues/8). Which make me think it's not as unobtrusive as advertised 😆.

Should we adopt gitlab-styles instead to at least have consistency across GitLab? We could be a little stricter on some things in this project, like we already are thanks to rufo.