Update GitLab Shell to v8.1.1 to fix regressions

What does this MR do?

Updates GitLab Shell to v8.1.1.

This fixes a couple of regressions introduced in GitLab Shell v8.0.0.

Since v11.2-rc4 currently uses 8.1.0, we'll need to pick this into 11-2-stable.

See gitlab-shell!226 (merged) for full details of the regression.

What are the relevant issue numbers?

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