MSVC: don't opt into dyndep support

Instead, just set the variables for how scanning works since that is
part of the compiler mechanisms.

Fixes: #24198

.gitlab/ci/cxx_modules_rules_msvc.cmake

@@ -1 +1,3 @@
 set(CMake_TEST_CXXModules_UUID "a246741c-d067-4019-a8fb-3d16b0c9d1d3")
+
+set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP 1)

Help/dev/experimental.rst

@@ -41,7 +41,8 @@ to tell CMake how to invoke the C++20 module dependency scanning tool.
 
 MSVC 19.34 (provided with Visual Studio 17.4) and above contains the support
 that CMake needs and has these variables already set up as required and only
-the UUID variable needs to be set.
+the UUID and the ``CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP`` variables need to be
+set.
 
 For example, add code like the following to a test project:
 

Modules/Compiler/MSVC-CXX.cmake

@@ -79,7 +79,6 @@ elseif (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.0)
 endif()
 
 if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL "19.34")
-  set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP 1)
   string(CONCAT CMAKE_EXPERIMENTAL_CXX_SCANDEP_SOURCE
     "<CMAKE_CXX_COMPILER> <DEFINES> <INCLUDES> <FLAGS> <SOURCE> -nologo -TP"
     " -showIncludes"