This removes the RemovedInReviewBoard40Warning class, and introduces

RemovedInReviewBoard60Warning and RemovedInReviewBoard70Warning.
RemovedInReviewBoard50Warning still remains, as we haven't removed the

deprecated code using it yet.
RemovedInNextReviewBoardVersionWarning has been set to

RemovedInReviewBoard60Warning.

Unit tests pass.
Used RemovedInReviewBoard60Warning in new in-progress code.