Update the `COM_URL` related links configurations

Currently the value of COM_URL in https://gitlab.com/gitlab-jh/gitlab/-/blob/main-jh/lib/gitlab.rb is still https://gitlab.com, we need to update it to https://gitlab.cn so that it can be used to check if it is a JH SaaS env or JH self-managed env. See https://gitlab.com/gitlab-jh/customers-gitlab-cn/-/issues/4 for more discussions.