[BUILD] Bump cmake version to 3.9 (#2581)

Author: marcalff

CMakeLists.txt

@@ -1,7 +1,7 @@
 # Copyright The OpenTelemetry Authors
 # SPDX-License-Identifier: Apache-2.0
 
-cmake_minimum_required(VERSION 3.1)
+cmake_minimum_required(VERSION 3.9)
 
 # See https://cmake.org/cmake/help/v3.3/policy/CMP0057.html required by certain
 # versions of gtest