Resolve "Geo: Unable to disable verification on the secondary"

What does this MR do?

Use string rather than a symbol as feature flag name on Gitlab::Geo.repository_verification_enabled?.

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

No.

Why was this MR needed?

Unable to disable verification on the secondary 😞.

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #6964 (closed)