Fix up deprecation classes for 6.0.
Review Request #12690 — Created Oct. 19, 2022 and submitted
A merge seems to have ended up creating duplicate 6.0 deprecation
classes. We also had a bad comment in here that referred to Djblets
instead of Review Board.
Ran unit tests.
line too long (80 > 79 characters) Column: 80 Error code: E501