Switch to backend a11y comparison

What does this MR do?

The frontend previously handled the comparison of the accessibility reports. We are switching this to happen on the backend.

To handle this, we need to

• change our endpoint to be a single endpoint
• poll the endpoint while the comparison happens
• update the schema to match the backend response
• update the respective specs

This change is behind a feature flag, and no changelog 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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done