Disable invalid_scan_result_policy_prevents_merge by default

What does this MR do and why?

This reverts merge request !119609 (merged) and sets the feature flag invalid_scan_result_policy_prevents_merge back to disabled by default. Due to &10539 (closed), we decided to keep this flag disabled by default for %16.0 and gather more feedback before releasing it in %16.1.

The documentation changes related to the feature flag were reverted to the state before: !119900 (merged)

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