[SPIKE] Evaluate deprecation of `allow_multi_rule` in approval rules

Context

multiple_approval_rules feature was added since the very beginning of approval rules. Now that approval rules have a CE/EE differentiation we should evaluate if this logic could be deprecated or modified.

For more details see https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/4812#note_2381044690 (internal only).

Considerations

There are workflows depending on this logic that affect both frontend and backend components.