Support `.ruby-version`

In https://github.com/rails/rails/issues/51089 the `.ruby-version` file is being adopted by Rails in Rails 8. They're also dropping support for integrating it with bundler by default. To that end we can add support for the `.ruby-version` file but it's unclear which should "win" when there's a conflict.

Here's an example implementation of the `.python-version` https://github.com/heroku/buildpacks-python/pull/272

The logic should be like this:

- If there is a Ruby version in `Gemfile.lock`, use it. Otherwise use `.ruby-version` otherwise Error.
- If there's a ruby version in both the Gemfile.lock and the .ruby-version error if they disagree

Known tasks:

- [ ] Add support for `.ruby-version` file as described above
- [ ] Research and document syntax of most common tools for `.ruby-version`
- [ ] Add JVM detection to also look at the .ruby-version file