Use gitlab-workhorse 6.0.0

What does this MR do?

Updates the bundled gitlab-workhorse version from 5.1.0 to 6.0.0. Changes:

v 6.0.0

• Accelerate Maven artifact repository uploads

v 5.2.0

• Populate Git Protocol
• Add support for GitConfigOptions required for git-receive-pack command

These are all feature additions. The major version changed because we also deprecated some internal workhorse <-> rails API that is obsolete since GitLab 10.7.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides