Improve SSH database key lookup documentation

What does this MR do?

Updates GitLab Shell to a version that contains a new helper script, and updates the instructions to make use of it

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

Database key lookups are currently an EE-only feature, but are being ported to CE in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16014

To keep things simple, I've based this MR atop the EE->CE backport MR.

Why was this MR needed?

Simplify database key lookups, specifically for ~Geo

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Has been reviewed by Documentation
• 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?

Related to gitlab-shell#118 (closed) omnibus-gitlab#2855 (closed)

Closes #41399 (closed)