zephyr: Increase minimum CMake version to 3.20.0.

As a prerequisite to upgrading to Zephyr v2.7.0, upgrade the minimum
CMake version required for the Zephyr port to 3.20.0.
Signed-off-by: Maureen Helm <maureen.helm@intel.com>

ports/zephyr/CMakeLists.txt

@@ -22,7 +22,7 @@
 # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 # THE SOFTWARE.
 
-cmake_minimum_required(VERSION 3.13.1)
+cmake_minimum_required(VERSION 3.20.0)
 
 find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
 project(micropython)