FindBoost: Add support for Boost 1.81

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

Output from BoostScanDeps.cmake is not changed comparing with the last version, so there are no new blocks with dependencies:

cmake -DBOOST_DIR=/data/data/com.termux/files/usr/include -P ~/cmake/Utilities/Scripts/BoostScanDeps.cmake
-- Scanning /data/data/com.termux/files/usr/include
-- set(_Boost_CONTRACT_DEPENDENCIES thread chrono)
-- set(_Boost_COROUTINE_DEPENDENCIES context)
-- set(_Boost_FIBER_DEPENDENCIES context)
-- set(_Boost_IOSTREAMS_DEPENDENCIES regex)
-- set(_Boost_JSON_DEPENDENCIES container)
-- set(_Boost_LOG_DEPENDENCIES log_setup filesystem thread regex chrono atomic)
-- set(_Boost_MATH_DEPENDENCIES math_c99 math_c99f math_c99l math_tr1 math_tr1f math_tr1l)
-- set(_Boost_MPI_DEPENDENCIES serialization)
-- set(_Boost_MPI_PYTHON_DEPENDENCIES python${component_python_version} mpi serialization)
-- set(_Boost_NUMPY_DEPENDENCIES python${component_python_version})
-- set(_Boost_THREAD_DEPENDENCIES chrono atomic)
-- set(_Boost_TIMER_DEPENDENCIES chrono)
-- set(_Boost_WAVE_DEPENDENCIES filesystem serialization thread chrono atomic)
-- set(_Boost_WSERIALIZATION_DEPENDENCIES serialization)

Fixes: #24255 (closed)
Backport: release
Topic-rename: FindBoost-1.81