Use gitlab-workhorse 3.4.0

What does this MR do?

Update the required version of gitlab-workhorse to 3.4.0.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Workhorse used to perform a repository existence check on disk when handling a Git HTTP request. This makes no sense anymore with Gitaly, and it is problematic for our Cloud Native project.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes gitaly#886 (closed)