Upgrading a trivial Rails app from Ruby 2.3.1 to 2.7 and from Rails 4.2.6 to 6.0.3

This blogpost describes the upgrading process of a trivial Rails app. The app is called PlusOne (it’s open-sourced) and it’s a small but fun Slack bot (MS Teams support coming).This app doesn’t rely on any external gems, consists only of 3 database tables. It just stores who gave upvote to whom. Additionally it shows some stats - who has how many points.Upgrading Ruby and Rails for such no-external-gems apps is mostly trivial, but there are some gotchas which I have documented along the way.The actual initial trigger for those upgrades was the enforcement of Heroku to upgrade the stack. This required newer Ruby, so here we are.