CI: Get to green

[olleolleolle]

This PR is a long quest to get to green in this repo.

The thing that was most broken was:

• Appraisal not having added the ruby platform (so I added it)
• RuboCop having old configuration (so I separated it into its own Lint CI job)
• The gem currently does not support Rails 8.0 - the tests do produce output, but the expectations fail.
• The gem currently does not supporting Rails 8.1 LogSubscriber new API

Learnings and notes

Appraisal offers calling commands on all the gemfiles at once: very practical. Example:

appraisal bundle update --bundler

[brunofacca]

Thank you very much for this big contribution!
This looks good to me in the sense that it's a big step in the right direction (3 Ruby/Rails version combinations are now passing). Would you like me to merge as is (after the git conflicts are fixed) or are you planning on pushing any additional commits?

[olleolleolle]

@brunofacca Um, I'll fix the conflicts (EDIT: ✅ now done).

I intend no further changes to this PR.

[brunofacca]

Thank you again, we finally have a working CI after Travis discontinued their free tier!