Add a default for the version option

[andrewn617]

Publishing the feature failed with some schema validation errors: https://github.com/rails/devcontainer/actions/runs/8116445483/job/22186482114#step:3:71

It seems that the github action expects there to be either an enum, proposal or default in for each of the children in the options array. (source)

However, this seems wrong because both the docs and the implementation on the devcontainer cli list them as optional. I should be able to have a string option with no default and no enum / proposal options.

I want to avoid listing all the options since we will have to release a new version of the feature for every patch ruby version.

So for now it seems the best option is to add a default, altho longterm I don't think we want it. People should be explicit about the ruby version always IMO.

I will open an issue / PR to the github action to try to fix this issue. But just want to get something working for now.