Use shipping runtime.json for RID targeting (#80074)

Use shipping runtime.json for RID targeting (#80074)

* Use shipping runtime.json for RID targeting

- Use the shipping runtime.json file to target RIDs when building
  instead of the custom OSGroups.json which only contained a subset of
  the available rids.
- To facilitate that, lower-case the existing platforms in the target
  frameworks strings which is the expected format anyway by the SDK and
  msbuild. This will eventually make it possible to bring some of the
  custom build infrastructure back into the SDK.
- Remove a few outdated code pieces and update docs.