Test updating SaaS URL in staging env

Problem to solve

We will officially change our SaaS URL from gitlab.cn to jihulab.com by the end of Dec., but before doing that, we'd like to try it in staging env first and figure out if there are any issues so that there will be no surprise when we do it in production env.

Proposal

1. @memorycancel Change SaaS URL to jihulab.com in a development branch, see https://gitlab.com/gitlab-jh/gitlab/-/merge_requests/28 as a reference.
2. @vincent_stchu Build packages & images with the branch created in step 1.
3. @vincent_stchu @chih7 Upgrade staging env with the packages & images created in step 2 and with the new URL staging.jihulab.com (see https://gitlab.cn/jihulab/jh-infra/production/-/issues/62 for more details), and then check if there is any issues. Note: we can not do this step when we are applying domain name filling for jihulab.com.
4. @holly.yang change all shared-runners register url

/cc @shreychen @holly.yang