Remove mail_room from Gemfile
In Omnibus, mail_room will soon be installed as its own software component (omnibus-gitlab!5122 (merged)) because its dependencies may differ from Rails (e.g.
net-imap v0.2.1, which is now a default gem in Ruby 3.0).
We did this because the latest version of the digest gem has a bug where the gem attempts to install
digest.h in the system directory. This fails if the user does not have permission: https://github.com/ruby/digest/issues/14.
Eventually, we may want to bring this back to the Gemfile since mail_room does depend on the GitLab config. However, this means source installs won't have the latest version of
gitlab-mail_room unless they manually override