Remove API version from GitLab extractor API URL

Problem

GitLab has removed API v3 support in GitLab 11.0 and future API versions that will also include GraphQL support will be based on the same version for compatibility reasons.

This allows for a stable API endpoint, but also means new features can be added to the API in the same version number.

1. This means that including the API version in the GitLab extractor configuration is redundant.
2. Requiring the base URL is also common for GitLab when adding a Kubernetes cluster.

Proposal

Let's drop the API version altogether from the API URL within the GitLab extractor configuration.