Update target of CMake to 3.8 for Rolling (ros2#2326)

If I create ros2 package with this in ros2 rolling --> ros2 pkg create --build-type ament_cmake --node-name hello_world_node hello_world

default CMakeLists.txt target is 3.8. I think, it is obsolete in Code style

Author: hodnajit

source/Contributing/Code-Style-Language-Versions.rst

@@ -348,7 +348,7 @@ CMake
 Version
 ^^^^^^^
 
-We will target CMake 3.5.
+We will target CMake 3.8.
 
 Style
 ^^^^^