Update GitLab-Workhorse to 7.1.0

What does this MR do?

Update workhorse to v 7.1.0

• Add structured logFormat for text based logging gitlab-workhorse!275 (merged)
• Run make fmt on master gitlab-workhorse!306 (merged)
• Allow to configure BUILD_DIR and TARGET_DIR gitlab-workhorse!308 (merged)
• Resolve "Rework test suite to allow dead code to be removed" gitlab-workhorse!307 (merged)
• Update Prometheus vendoring gitlab-workhorse!305 (merged)
• General vendoring cleanup gitlab-workhorse!310 (merged)
• Remove Go 1.8 support gitlab-workhorse!314 (merged)
• Remove unused 'body' argument gitlab-workhorse!315 (merged)
• Refactor badgateway to use standardlib interfaces gitlab-workhorse!316 (merged)
• Pass Correlation-Ids down to backend systems gitlab-workhorse!311 (merged)
• Don't fail if /home/git/repositories already exists in Gitaly container gitlab-workhorse!317 (merged)