FindBoost: Clarify role of legacy variables in warning message

Revise the wording of the warning added by commit 0dd6772a89 (FindBoost:
Add legacy variables and targets for compatibility, 2019-06-14,
v3.15.0-rc2~3^2~1) to refer to the old variables as "legacy" rather
than "standard".

Suggested-by: Alexander Grund <[email protected]>
Issue: #19402

Modules/FindBoost.cmake

@@ -345,7 +345,7 @@ function(_boost_set_legacy_variables_from_config)
       _boost_get_existing_target(${comp} target)
       if(NOT target)
         if(Boost_DEBUG OR Boost_VERBOSE)
-          message(WARNING "Could not find imported target for required component '${comp}'. Standard variables for this component might be missing. Refer to the documentation of your Boost installation for help on variables to use.")
+          message(WARNING "Could not find imported target for required component '${comp}'. Legacy variables for this component might be missing. Refer to the documentation of your Boost installation for help on variables to use.")
         endif()
         continue()
       endif()