Use gitlab-shell 6.0.2

What does this MR do?

Update the required gitlab-shell version from 5.11.0 to 6.0.2.

This the changelog:

v6.0.2
  - Use grpc-go 1.9.1 (!184)
  - Update gitaly-proto and gitaly libs (!185)

v6.0.1
  - Fix git push by removing a bad require in the pre-receive hook (!183)

v6.0.0
  - Remove bin/gitlab_projects (!180)
  - Remove direct redis integration (!181)
  - Remove support unhiding of all references for Geo nodes (!179)

The removals in 6.0.0 are technically breaking changes, but they remove code that has been unreachable since GitLab 10.4 or earlier. So GitLab 10.5 should be a safe time to remove that unreachable code.

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

Why was this MR needed?

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?

gitaly#937 (closed)