Follow-up: convert pending members endpoint to GraphQL

Proposal

In #332596 (closed) we've added the pending_members endpoint.

We'd like to have the endpoint as GraphQL.

The issue is a follow-up from !74814 (comment 760706916).

Add the GraphQL endpoint
Update frontend to use GraphQL instead of old endpoint in ee/app/assets/javascripts/api/groups_api.js
~~Remove the old pending_members endpoint if it's not being used anymore.~~ (see https://docs.gitlab.com/ee/api/rest/#compatibility-guidelines)

Edited May 22, 2024 by Missy Davies