Restore feature flag for legacy merge request state check

What does this MR do and why?

This MR introduces a feature flag to selectively disable the fixed behavior.

Originally, we went through a fix and feature flag rollout process in !86612 (merged) and !89951 (merged), however, it seems there are still a few users that needs time to adjust their workflow. For those users, we can restore the previous behavior on specific projects so that they can buy more time to adjust their workflow.

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