Starting with Boost 1.70, upstream Boost provides an official CMake Package Configuration file that replaces the need for a FindBoost module. !3243 and !3438 add some compatibility logic to FindBoost to aid the transition. The latter adds the legacy variables even when the BoostConfig.cmake file does not provide them. Projects should port away from the variables and eventually away from FindBoost completely. Eventually we should add a policy to hide FindBoost altogether and pretend that it does not exist so that `find_package(Boost)` will only search for a `BoostConfig.cmake` file. We already did this with [CMP0084](https://cmake.org/cmake/help/v3.15/policy/CMP0084.html) for `FindQt`. However, using the policy's NEW behavior means that a project will never find a version of Boost that does not provide a `BoostConfig.cmake` file. Since polices are not feature toggles and should generally not be set to `OLD`, such a policy will make sense only when versions of Boost prior to 1.70 have fallen out of use.