Fix the lock-yourself-out prevention code and spec for Geo

What does this MR do?

Geo is supposed to prevent you from adding the primary node to the database as a secondary. However, the validation implementing this was not running, and the spec was broken too.

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
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #3506 (closed)