Make omnibus-mirror group in dev public

For gitlab-org/omnibus-gitlab#3812 (closed), we need to have a modified ohai gem (which can detect openSUSE Leap 15.0 packages). We have a few PRs submitted upstream for this, but if upstream doesn't cut a new release with our PRs soon, we will have to install it from a fork - which usually goes into https://gitlab.com/gitlab-org/build/omnibus-mirror group and gets mirrored to https://dev.gitlab.org/omnibus-mirror.

However, unlike our other libraries that we mirror, we need ohai not only during package building, but during bundle install (which is done in a global before_script) step in our CI. SSH access to dev is limited only to package building jobs, not the ones run on gitlab.com. So, ohai mirror needs to be a public project, similar to https://dev.gitlab.org/gitlab/omnibus. However, making this project public means making that whole group public.

We don't host anything confidential or sensitive in that group. It is mainly intended as a backup for us to build packages without depending on third party repos. So, I don't think there is any problem in making that group public.

@gitlab-org/distribution WDYT?

cc @marin