We've been running Gerrit 3.7 for some time now and seem very unlikely
to revert at this point. Clean up the Gerrit 3.6 image builds as we
don't need them anymore.
THis change also comments out the 3.6 -> 3.8 upgrade job. Followup
changes will add 3.8 image builds and test the 3.7 -> 3.8 upgrade
process.
Depends-On: https://review.opendev.org/c/openstack/project-config/+/881595
Change-Id: I759b34e48dcede7ffaa66c83da01b81c4fed4b4f