Add deprecation warning for centos6

Add a deprecation warning to https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/c11977a50002e9d415a6fb7773c42a7d66f3d0b2/files/gitlab-cookbooks/package/libraries/omnibus_helper.rb#L101 for centos6

This is in preparation for the centos6 removal here: #5163 (closed)