build: set zmk_config.cmake cmake_minimum_required to VERSION 3.15
3.15 is the minimum version required to facilitate list(PREPEND) used within zmk_config.cmake.
Fixes
# * single overlay,
# * or per board/shield.
+cmake_minimum_required(VERSION 3.15)
+
get_property(cached_user_config_value CACHE ZMK_CONFIG PROPERTY VALUE)
set(user_config_cli_argument ${cached_user_config_value}) # Either new or old
