Update `pika_setup_boost.cmake` to use Boost's built-in CMake configuration

[msimberg]

CMake 3.30 deprecates CMake's FindBoost modules and attempts to use Boost's CMake configuration. We can fall back to the old policy temporarily, but long term we should prefer to use Boost's own configuration. See https://cmake.org/cmake/help/latest/module/FindBoost.html.

I briefly attempted to use Boost's own configuration in a spack install tree, but curiously I could not see a BoostConfig.cmake file in the install tree, so we'd need to check if it's just my installation that was missing it or if we need to take that into account more generally.