This makes two major improvements to how we generate this page:

- It's entirely dynamic. We no longer have to define which versions we
  want, or what month to start on.
- It's entirely local. We no longer hit the live site and parse the
  HTML. All of the data we need is in this repository, after all.

This was also the only place using the `oga` gem, so we've removed that.
One less gem to install! 🎉