See https://www.mediawiki.org/wiki/GitLab_consultation
TODO:
- Resolve nearly all subtasks of T330797 (note: migrating canonical Diffusion repos is not a hard blocker - after uninstalling Differential folks can still clone Diffusion repos but they cannot commit into Diffusion repos anymore)
- First set Can Use Application to Administrators on https://phabricator.wikimedia.org/applications/view/PhabricatorDifferentialApplication/
- Uninstall in the UI via https://phabricator.wikimedia.org/applications/view/PhabricatorDifferentialApplication/
- Remove from default settings via https://gitlab.wikimedia.org/repos/phabricator/deployment/-/merge_requests/28
- Deploy new default settings
- Check that "Local Config" at https://phabricator.wikimedia.org/config/edit/phabricator.uninstalled-applications/ reflects this