Support `.ruby-version`

[schneems]

In rails/rails#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 heroku/buildpacks-python#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