We'll want to merge this when we're confident that we won't be reverting
Gerrit back to 3.7 from 3.8. A followup will add Gerrit 3.9 images and
reenable the upgrade job.
Depends-On: https://review.opendev.org/c/openstack/project-config/+/901465
Change-Id: I26b5afadbe271cb6abea5b8bd79fa8a4a30ea478