COMP: Fix CMake configuration for cmake versions without CMP0135

Fixes compilation error introduced by commit
7972041 for versions prior to CMake 3.24:

CMake Error at Utilities/ClangFormat/DownloadClangFormat.cmake:47 (cmake_policy):
  Policy "CMP0135" is not known to this version of CMake.
Call Stack (most recent call first):
  CMake/ITKModuleClangFormat.cmake:17 (include)
  CMake/ITKModuleMacros.cmake:9 (include)
  CMakeLists.txt:146 (include)

Utilities/ClangFormat/DownloadClangFormat.cmake

@@ -43,8 +43,10 @@ endif()
 if(ITK_FORBID_DOWNLOADS)
   message(SEND_ERROR "Attempted to download ClangFormat when ITK_FORBID_DOWNLOADS is ON")
 endif()
-cmake_policy(PUSH)
-cmake_policy(SET CMP0135 NEW)
+if(POLICY CMP0135)
+  cmake_policy(PUSH)
+  cmake_policy(SET CMP0135 NEW)
+endif()
 if(NOT TARGET ClangFormat AND _clang_format_hash)
   ExternalProject_Add(
     ClangFormat
@@ -60,4 +62,6 @@ if(NOT TARGET ClangFormat AND _clang_format_hash)
     LOG_INSTALL 0
     INSTALL_COMMAND ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_BINARY_DIR}/ITKClangFormatConfig.cmake)
 endif()
-cmake_policy(POP)
+if(POLICY CMP0135)
+  cmake_policy(POP)
+endif()