Update foreman-debug manpage and remove mailing list refs
As per discussion on the linked Bugzilla, it was discovered that:
1. In the man page, -u parameter should be more specific about where its uploading debugs to.
2. After upload foreman-debug prints "The tarball has been uploaded, please contact us on our mailing list or IRC". It can be more specific about which mailing list/IRC the users are being asked to contact us on.
3. Foreman-users mailing list has been deprecated upstream. Users may be directed to the community forum.