Update Pages to use -domain-source=gitlab by default
The -domain-source configuration flag for GitLab Pages was introduced with gitlab#217912 (closed) to incrementally rollout use of GitLab API-based source configuration for Pages.
TODO
Set -domain-source=gitlab as the default value for GitLab Pages in %13.7
Follow up from #5350 (closed)
/cc @jmeshell @sean_carroll @vshushlin
Edited by Jaime Martinez