Update Workhorse to 7.0.0 for Gitaly's new auth scheme

What does this MR do?

Bring the workhorse version that uses the new gitaly auth scheme

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitaly/issues/1319 (since it's the last component missing)

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