Follow-up from "Adjust findings finder to remove feedback dependency based on feature flag"

The following discussion from !105097 (merged) should be addressed:

• @minac started a discussion: (+4 comments)

  Hi @morefice 👋 Would you mind giving me the initial database review on this?

Implementation Plan

• Update Security::FindingsFinder#all_security_findings to do only merge(Security::Scan.latest_successful) to avoid redundant second join instead of using .latest scope
• Remove the .latest scope from the Security::Finding model(make sure we are not using it anywhere else)