Finalise v2 MR level rules when MR is merged

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Why

Finalizing approval rules is currently done here https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/approval_rules/finalize_service.rb where it seems both MR and Project level rules are handled.

We need a way to finalize v2 MR rules.

What

This may require separating or decoupling finalization for project/MR level first.

It may require some investigatory code spike to get the right path forward. One of the things to decide will be whether we want to separate services out for v2 (in general, and in particular here), or try to inject the logic behind the FF in the existing service.