FindBoost: Add Boost 1.81 support

Update the list of known versions.

There is a new header-only library, Boost.URL. It has no
dependencies and has a core header `<boost/url.hpp>`.

Run the command

    cmake -DBOOST_DIR=/path/to/boost_1_81_0 \
      -P Utilities/Scripts/BoostScanDeps.cmake

to extract dependencies from the 1.81.0 source tree.
They are the same as 1.80's dependencies, so just update
the version check for warning about newer versions.