Add Support For GitLab API v4

[MarshTheBacca]

Using a private instance of GitLab, this extension is unable to make a valid API request. The server responds with the following:

[GitHubServer] Received fallback response from the server: {"error":"API V3 is no longer supported. Use API V4 instead."}

This is repeated dozens of times when opening the repository and when attempting to create a pull request.