mail_room.rb

This only has minor fixes but allows for the v2 of oauth2 gem and gets
rid of an unnecessary io-wait gem dependency, which will need to be
updated for Ruby 3.1+. The io-wait version was pinned due to
https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/6471, which has
now been resolved upstream
(https://github.com/ruby/io-wait/commit/1060f9348c60a62e4a5e00e718a605cef00f7bbd).

Diff: https://my.diffend.io/gems/gitlab-mail_room/0.0.20/0.0.23

Changelog: changed