[cmake][dataflow] update cmake min version to 3.18

Author: MathiasPaulin

examples/DataflowExamples/GraphRendering/CMakeLists.txt

@@ -1,7 +1,6 @@
-cmake_minimum_required(VERSION 3.6)
-if(${CMAKE_MAJOR_VERSION}.${CMAKE_MINOR_VERSION} VERSION_GREATER "3.9")
-    cmake_policy(SET CMP0071 NEW)
-endif()
+cmake_minimum_required(VERSION 3.18)
+cmake_policy(SET CMP0071 NEW)
+
 if(APPLE)
     cmake_policy(SET CMP0042 NEW)
 endif(APPLE)