Update or retire ROADMAP.md

**System Information (please complete the following information):**
- N/A

**Describe the bug**
https://github.com/dotnet/machinelearning/blob/main/ROADMAP.md is oriented around ML.NET 3.0 and may confuse others or look bad as this release has already occurred. This file comes up in SEO for searches for ML.NET 4.0 or ML.NET upcoming features.

**To Reproduce**
See https://github.com/dotnet/machinelearning/blob/main/ROADMAP.md

**Expected behavior**
I'd love this page to get updated periodically, but barring that it should probably be removed

**Screenshots, Code, Sample Projects**

**Additional context**
Was looking for resources to link to for ML.NET 4.0 and what to expect from ML.NET ahead for an article I'm writing.