kde/kf6-extra-cmake-modules: rebase to 6.28.0 (ECM) + teach version parser set(VERSION)

kf6-kcoreaddons 6.28.0 requires ECM >= 6.28.0 but the vendored ECM source/ was
still 6.10.0: the sync engine skipped it because ECM's version marker is a bare
set(VERSION "X.Y.Z") that the parser didn't recognize (only KF_VERSION /
PROJECT_VERSION / RELEASE_SERVICE_VERSION). Re-lay ECM from its 6.28.0 source.tar
(pure cmake modules, no Redox port) and teach both the sync engine and the
validator to read set(VERSION) so this class is caught, not silently skipped.

local/recipes/kde/kf6-extra-cmake-modules/source/modules/ECMGenerateDBusServiceFile.cmake

@@ -56,6 +56,8 @@ Example usage:
 Since 5.73.0.
 #]=======================================================================]
 
+cmake_policy(VERSION 3.16)
+
 function(ecm_generate_dbus_service_file)
     set(options)
     set(oneValueArgs EXECUTABLE NAME SYSTEMD_SERVICE DESTINATION RENAME)