Replace `Security::FindingsFinder` with `Security::PureFindingsFinder`

Why are we doing this work

After running this migration, we can replace the Security::FindingsFinder with Security::PureFindingsFinder. We want to keep the name Security::FindingsFinder but replace its logic with Security::PureFindingsFinder.

Implementation plan

1. Move Security::PureFindingsFinder code and tests into Security::FindingsFinder.
2. Rename Security::PureFindingsFinder references to Security::FindingsFinder.

Verification steps

• TBD