Update GitLab Shell to v8.4.0 for new Gitaly auth scheme

What does this MR do?

Update GitLab Shell to v8.4.0, which uses Gitaly auth scheme v2

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitaly/issues/1319

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