Run Rubocop in GitLab CI

On #17646 (closed) we built a Rubocop configuration that was, at the time, happy with our Ruby code base. This required updating tons of our Ruby code that was written in a style that diverged greatly from best practices. For a while it helped me quite a bit. But nowadays it does not help that much: we've never enforced "no regression on Rubocop checks" in any way, so unsurprisingly we did introduce dozens of regressions, and we're back to a state where I can't assume anymore that any Rubocop violation in code I'm working on is a regression I've introduced.

To fix this, I think we should enforce Rubocop in GitLab CI.

It's possible we hit difficulties due to different versions of Rubocop on our developers' systems (some use Debian stable, other use Debian unstable) and on GitLab CI. I'm not too worried though: Rubocop has decent versioning support.