Lacking documentation on change in javascript runtime installation behavior between classic and current ruby buildpack

[lilacstella]

The ruby buildpack failed to automatically build in JRuby in heroku/jruby-getting-started#26. This is due to the lack of a javascript runtime when building without the Node.js buildpack, there should be documentation regarding that when users upgrade from the classic version to CNBs. More details in #295.

We should add some clarification in https://github.com/heroku/buildpacks-ruby/blob/main/docs/upgrading.md.