Update GitLab Workhorse to v5.1.0

What does this MR do?

Updates GitLab Workhorse to v5.1.0

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

Changes are listed here: gitlab-workhorse#178 (closed)

Why was this MR needed?

Requested by @andrewn for the prometheus changes specifically

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
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Related to gitlab-workhorse#178 (closed)