See https://www.mediawiki.org/wiki/GitLab_consultation
TODO:
[x] 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)
[x] First set {nav Can Use Application} to {nav Administrators} on https://phabricator.wikimedia.org/applications/view/PhabricatorDifferentialApplication/
{F36885716 size=full}
[x] 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
[ ] Check that "Local Config" at https://phabricator.wikimedia.org/config/edit/phabricator.uninstalled-applications/ reflects this