After [we consolidated Group and Project member management](https://gitlab.com/gitlab-org/gitlab/-/issues/337104) and only have a single `NamespaceMember` type, many of the policies will be redundant. Eg. instead of `read_group` and `read_project`, we only need `read_namespace`. We should refactor `GroupPolicy` and `ProjectPolicy` to `NamespacePolicy` to reduce duplication. The majority of this work will be blocked by Members moving to Namespaces https://gitlab.com/groups/gitlab-org/-/epics/7054.