We introduced mail_room as a separate dependency in GitLab Omnibus
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
(gitlab-com/gl-infra/production#4053).

We've updated to Ruby 3.0.5 for Cloud Native GitLab images now and
have already bumped the net-imap dependency in
!115755. As a result,
we can safely update mail_room in `Gemfile` again.

omnibus-gitlab!6804
will drop mail_room as a separate dependency, so there's one fewer
place to update the version.

Changelog: changed