* `member_role_id` is the attribute used to associate a member record
  with a custom role (member_role)
* By adding this param to the PUT endpoint, we are enabling:
  * associating *group* members with custom roles
  * associating *project* members with custom roles
  * de-associating group/project members from a custom role (removing a custm role
    from a user)
* Custom roles is a feature available to Ultimate customers and is
  currently behind the `customizable_roles` feature flag.
* A previous MR added a `PATCH /groups/:id/members/:user_id/member_role`
  endpoint for associating existing group members with a custom role.
* The PATCH endpoint can be removed after this is merged because that
  behavior is now covered by the existing endpoint.
* https://gitlab.com/gitlab-org/gitlab/-/issues/387769
* https://gitlab.com/gitlab-org/gitlab/-/issues/385495

Changelog: added
EE: true