Start a new Duo review on push only for DAP reviews

What does this MR do and why?

Automatic Duo Code Review can start a new review on every push. Only Duo Agent Platform (DAP) reviews read the earlier discussions on a merge request, so a classic review on every push repeats the comments of the review before it. This merge request limits review on push to DAP reviews. Classic reviews keep the old behavior: one automatic review on the first push with changes.

The project and group settings now hide the "Start a new review on push" checkbox when the Code Review foundational flow is not available. The admin setting stays, with help text that says it applies to DAP reviews. The classic Duo Code Review docs no longer describe the setting.

References

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Before	After

How to set up and validate locally

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