[SPIKE] Evaluate deprecation of `allow_multi_rule` in approval rules
## Context `multiple_approval_rules` feature was added since the [very beginning of approval rules](https://gitlab.com/gitlab-org/gitlab/-/commit/f480d1455da1cffae21a17f204a8e45c861b5d27). 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.
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