Publish .NET images as OCI formats instead of Docker formats

### Describe the Problem

The .NET base images today are publishes to MCR as Docker Manifest Lists comprised of individual Docker Manifests. These formats are older, but over the years the OCI formats have become the commonly-accepted standard. .NET should embrace this change and produce OCI-compatible Image Indexes and Images for our base images.

### References

* [OCI Layout specification](https://github.com/opencontainers/image-spec)
* [oras CLI tool](https://oras.land/docs/)