Make methodology for new feature proposals consistent

[ottonomy]

We have some dangling (already completed) items still hanging out in ob_next. We separately have the proposals directory to document current, future, and prior proposals that are outside a specific version. We also use GitHub issues for feature requests and new use cases, which is great.

I propose we delete the ob_next folder and its contents as we direct people to put new feature requests in issues. As feature requests coalesce, we may make a proposal to check into the repo as a historical document that includes a description of the rough scope of a new spec version for OB and/or CLR and the combined data model.