Allow Product Managers to conditionally waive Multi-Version Compatibility constraint

Why is this change being made?

Related to gitlab-org/gitlab#352455 which has some good discussions for understanding context.

https://gitlab.com/gitlab-data/product-analytics/-/issues/801 gives a concise overview of the problem.

The development process with the strict constraints means incompatible code is never shipped, even if the risk is small. Product Managers are well placed to judge (with Engineering guidance) if that risk is acceptable. This MR adds this and explains the risk factors and provides examples of when risks are potentially mitigated (e.g. when behind feature flag).

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