fix: respect PYTHON_VERSION if set in classic mode (#2414)

* fix: respect PYTHON_VERSION if set in classic mode

* fix: add warning when using PYTHON_VERSION
diff --git a/tools/pybind11Common.cmake b/tools/pybind11Common.cmake
index f058451..8f7f57b 100644
--- a/tools/pybind11Common.cmake
+++ b/tools/pybind11Common.cmake
@@ -147,7 +147,7 @@
   else()
     set(supported_standards 11 14 17 20)
     if("${VAL}" IN_LIST supported_standards)
-      message(WARNING "USE -DCMAKE_CXX_STANDARD=${VAL} instead of PYBIND11_PYTHON_VERSION")
+      message(WARNING "USE -DCMAKE_CXX_STANDARD=${VAL} instead of PYBIND11_CPP_STANDARD")
       set(CMAKE_CXX_STANDARD
           ${VAL}
           CACHE STRING "From PYBIND11_CPP_STANDARD")