Use backend code quality diff for MR widget when feature flag is enabled

What does this MR do?

This MR switches the data source for the code quality MR widget: when the :codequality_mr_diff feature flag is enabled, the widget will use a diff generated by the backend. For more context on why we want to do this, see #271077 (closed).

Screenshots

There should be no visual changes, but for clarity, this is the widget that this change affects:

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry none due to feature flag
• [-] Documentation none due to feature flag
• 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