github client: Cleaner separation of concerns between v3 and v4

Created by: indradhanush

The GitHub V4 client (GraphQL) makes a REST API request to determine the version even though the GitHub V3 client (REST) exists in the codebase. This reflects a bad separation of concerns.

Ideally, the GitHub V4 client should internally use the V3 client to make this request. This would eliminate boilerplate in the V4 client while we already have it in the V3 client.