Stops group approvers from leaking private group information

...ntrollers/projects/merge_requests/creations_controller.rb

app/controllers/projects/merge_requests_controller.rb

app/controllers/projects_controller.rb

app/finders/joined_groups_finder.rb

app/models/group.rb

app/models/merge_request.rb

app/models/user.rb

app/views/projects/merge_requests/_form.html.haml

...s/projects/merge_requests/creations/_new_submit.html.haml

app/views/shared/issuable/_form.html.haml

ee/app/models/approver_group.rb

ee/app/presenters/ee/merge_request_presenter.rb

ee/app/presenters/ee/project_presenter.rb

ee/app/views/shared/issuable/_approvals.html.haml

...6881-project-group-approvers-leaks-private-group-info.yml

...c/features/projects/settings/service_desk_setting_spec.rb

ee/spec/models/approver_group_spec.rb

ee/spec/presenters/merge_request_presenter_spec.rb

ee/spec/views/shared/issuable/_approvals.html.haml_spec.rb

rubocop/cop/group_public_or_visible_to_user.rb