Move Review Board master to use Djblets 2.2+
Review Request #11672 — Created June 23, 2021 and submitted
Djblets master has had a number of changes to remove old compatibility
shims for older Django support, but Review Board master still tries to
import some of those. Until we have time to make equivalent updates
there, we still need to use 2.2.
Was able to set up and run Review Board master.