CMakeLists: Minimum version 3.14

Needed for NAME_WLE (see commit 9cdc4cf2104ba1592).
Thanks for Ilya Maykov for noticing.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7875504..6395bf3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 2.8.12)
+cmake_minimum_required(VERSION 3.14.0)
 
 project(nanopb C)