remove deprecated approved field

Federico Mannuccirequested to merge
gitlab-community/gitlab-org/api/client-go:fmannucci/deprecate-approved into release-client-3.0

Implementation of issue: #2278 (closed)

Remove deprecated and non functional "approved" field from go client

https://gitlab.com/gitlab-org/api/client-go/-/blob/main/merge_requests.go#L282

ApprovedByIDs=any can be used instead - documentation for reference

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