This builds off the documentation in
https://github.com/tpitale/mail_room/#microsoft-graph-configuration.

Omnibus support was merged in
https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/5145.

Related links:

1. https://gitlab.com/gitlab-org/gitlab/-/issues/214900
2. https://github.com/tpitale/mail_room/pull/125