Update minimum CMake version in CMakeLists.txt
# mbedtls, mbedx509, mbedcrypto and apidoc targets.
#
-cmake_minimum_required(VERSION 2.8.12)
+# We specify a minimum requirement of 3.10.2, but for now use 3.5.1 here
+# until our infrastructure catches up.
+cmake_minimum_required(VERSION 3.5.1)
# https://cmake.org/cmake/help/latest/policy/CMP0011.html
# Setting this policy is required in CMake >= 3.18.0, otherwise a warning is generated. The OLD
