Remove deprecated epic_iid from issues API in next major API version (V5)
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- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=35157)
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This is a follow-up of https://gitlab.com/gitlab-org/gitlab/issues/35089. It introduces a breaking change to the API and therefore needs to happen in a major API release (such as Version 5).
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