Move Review Board master to use Djblets 2.2+

Review Request #2190 — Created July 16, 2021 and updated — Latest diff uploaded

Review Board
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.

Testing Done:
Was able to set up and run Review Board master.

Reviewed at

I did stuff