Upgrade Gitaly to v1.7.1 for correlation-id logging

What does this MR do?

Upgrade Gitaly to v1.7.1 to support logging correlation-ids: gitaly!995 (merged)

Gitaly v1.7.1 changes: gitaly@v1.7.0...v1.7.1

What are the relevant issue numbers?

• gitaly!995 (merged)
• gitlab-workhorse!343 (merged)
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22844

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
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer