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.

Modules/FindBoost.cmake

@@ -1380,7 +1380,7 @@ function(_Boost_COMPONENT_DEPENDENCIES component _ret)
       set(_Boost_TIMER_DEPENDENCIES chrono)
       set(_Boost_WAVE_DEPENDENCIES filesystem serialization thread chrono atomic)
       set(_Boost_WSERIALIZATION_DEPENDENCIES serialization)
-      if(Boost_VERSION_STRING VERSION_GREATER_EQUAL 1.81.0 AND NOT Boost_NO_WARN_NEW_VERSIONS)
+      if(Boost_VERSION_STRING VERSION_GREATER_EQUAL 1.82.0 AND NOT Boost_NO_WARN_NEW_VERSIONS)
         message(WARNING "New Boost version may have incorrect or missing dependencies and imported targets")
       endif()
     endif()
@@ -1466,6 +1466,7 @@ function(_Boost_COMPONENT_HEADERS component _hdrs)
   set(_Boost_TIMER_HEADERS               "boost/timer.hpp")
   set(_Boost_TYPE_ERASURE_HEADERS        "boost/type_erasure/config.hpp")
   set(_Boost_UNIT_TEST_FRAMEWORK_HEADERS "boost/test/framework.hpp")
+  set(_Boost_URL_HEADERS                 "boost/url.hpp")
   set(_Boost_WAVE_HEADERS                "boost/wave.hpp")
   set(_Boost_WSERIALIZATION_HEADERS      "boost/archive/text_wiarchive.hpp")
   set(_Boost_BZIP2_HEADERS               "boost/iostreams/filter/bzip2.hpp")
@@ -1653,7 +1654,7 @@ else()
   # _Boost_COMPONENT_HEADERS.  See the instructions at the top of
   # _Boost_COMPONENT_DEPENDENCIES.
   set(_Boost_KNOWN_VERSIONS ${Boost_ADDITIONAL_VERSIONS}
-    "1.80.0" "1.80" "1.79.0" "1.79"
+    "1.81.0" "1.81" "1.80.0" "1.80" "1.79.0" "1.79"
     "1.78.0" "1.78" "1.77.0" "1.77" "1.76.0" "1.76" "1.75.0" "1.75" "1.74.0" "1.74"
     "1.73.0" "1.73" "1.72.0" "1.72" "1.71.0" "1.71" "1.70.0" "1.70" "1.69.0" "1.69"
     "1.68.0" "1.68" "1.67.0" "1.67" "1.66.0" "1.66" "1.65.1" "1.65.0" "1.65"