Update the deprecation classes used in Review Board.
Update the deprecation classes used in Review Board.

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`.

Testing Done:
Unit tests pass.

Used `RemovedInReviewBoard60Warning` in new in-progress code.

Reviewed at https://reviews.reviewboard.org/r/12327/

reviewboard/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard