Modify CNG to use gitlab-mail_room fork

mail_room has been forked for more expedient changes and pivots according to GitLab's specific needs.

MR: gitlab-org/gitlab!25869 (merged)

Omnibus MR: gitlab-org/omnibus-gitlab!3960 (merged)

CNG MR to use --log-exit-as: !395 (merged)

release-tools MR: gitlab-org/release-tools!884 (closed)