FindBoost: Update MinGW compiler tag for Boost 1.73

Boost 1.73 on MinGW no longer uses the compiler minor version in
its library names if the major version is at least 5.

Fixes: #20673

Modules/FindBoost.cmake

@@ -750,7 +750,11 @@ function(_Boost_GUESS_COMPILER_PREFIX _ret)
         set(_boost_COMPILER "-mgw") # no GCC version encoding prior to 1.34
     else()
       _Boost_COMPILER_DUMPVERSION(_boost_COMPILER_VERSION _boost_COMPILER_VERSION_MAJOR _boost_COMPILER_VERSION_MINOR)
-      set(_boost_COMPILER "-mgw${_boost_COMPILER_VERSION}")
+      if(Boost_VERSION_STRING VERSION_GREATER_EQUAL 1.73 AND _boost_COMPILER_VERSION_MAJOR VERSION_GREATER_EQUAL 5)
+        set(_boost_COMPILER "-mgw${_boost_COMPILER_VERSION_MAJOR}")
+      else()
+        set(_boost_COMPILER "-mgw${_boost_COMPILER_VERSION}")
+      endif()
     endif()
   elseif (UNIX)
     _Boost_COMPILER_DUMPVERSION(_boost_COMPILER_VERSION _boost_COMPILER_VERSION_MAJOR _boost_COMPILER_VERSION_MINOR)