lncfg: add deprecated no-experimental-endorsement config option

Re-adds the old no-experimental-endorsement config option as a hidden,
deprecated alias for no-experimental-accountability. This ensures
backward compatibility for users who have the old option in their
config files after the rename it.

sample-lnd.conf

@@ -1443,6 +1443,10 @@
 ; Set to disable experimental accountability signaling.
 ; protocol.no-experimental-accountability=false
 
+; DEPRECATED: Use protocol.no-experimental-accountability instead.
+; Set to disable experimental endorsement signaling.
+; protocol.no-experimental-endorsement=false
+
 ; Set to enable support for RBF based coop close.
 ; protocol.rbf-coop-close=false