Resolve "[Feature flag] Cleanup standalone_finding_modal"

What does this MR do and why?

It removes the standalone_finding_modal feature flag and all its usages.

Screenshots or screen recordings

No visible changes.

How to set up and validate locally

1. Go a pipeline security report
2. Click on findings and verify that the modal opens as expected

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #420372 (closed)