esp32/CMakeLists.txt: Require CMake version 3.12.
Because "find_package(Python3 ...)" requires at least this version of
CMake.  And other features like GREATER_EQUAL and COMMAND_EXPAND_LISTS need
at least CMake 3.7 and 3.8 respectively.
# Top-level cmake file for building MicroPython on ESP32.
-cmake_minimum_required(VERSION 3.5)
+cmake_minimum_required(VERSION 3.12)
# Set the location of this port's directory.
set(MICROPY_PORT_DIR ${CMAKE_SOURCE_DIR})
