Expose a11y report path

What does this MR do?

We're creating a merge request widget for the accessibility report. The comparison is done on the backend through and API request. This MR exposes that API endpoint, consistent with how the other MR widgets are receiving their API endpoint paths.

We're also fixing a minor bug with the frontend data. The backend is not currently returning warnings or notes, but will in future iterations. We're removing frontend references to this data for now.

There was also an extra feature flag that I had created for the frontend. Since backend already has a feature flag in place, we can remove this extra feature flag and avoid duplication.

This is behind a feature flag, so no changelog entry is included.

Related to #39425 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content