GitLab

Follow-up to https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77236.
This commit replaces all occurrences of the project/group specific
`project_member`/`group_member` methods with a generic `member` one,
being more in-line with the already existing `members` method. Both
methods were already aliased with a generic representations, which
is now no longer required.