gitlab.rb

We introduced mail_room as a separate dependency because updating
mail_room required an updated net-imap gem. This gem attempted to
install the digest gem, but due to a bug present in Ruby 2.7
(https://bugs.ruby-lang.org/issues/17761), the installation failed and
caused Cloud Native GitLab builds to fail
(https://gitlab.com/gitlab-com/gl-infra/production/-/issues/4053).

We've updated to Ruby 3.0.5 for Cloud Native GitLab images now and
have already bumped the net-imap dependency in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/115755. With
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/116494, we've
now updated GitLab Rails to use mail_room v0.0.23. As a result, we no
longer need a separate dependency for Omnibus.

Removing this dependency makes it easier to update mail_room since we
only need to bump the `Gemfile` in GitLab Rails.

Changelog: changed