Resolve "Bug: When using the /merge quick action, it returns 'Merged' even though it was not"

What does this MR do and why?

The current issue is that the merge orchestration code checks the mergeability_state, and not the mergeable_state. This means that these things are not being checked (https://gitlab.com/gitlab-org/gitlab/-/blob/08e1047a5ec8c90013cf2114f7d5b59e95fa9142/ee/app/models/ee/merge_request.rb#L122). If approvers, has_denied_policies or mr is blocked.

However, when we go to merge it later async, we check mergeable?. So the merge orchestration check can return true that it is mergeable, even though it is not.

We can either, a) change the mergeable_state to be mergeable, or move these three checks to mergeable_state.

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Related to #354453 (closed)