multi: rename experimental endorsement signal to accountable

Renames the endorsement signal to accountable to
match the latest proposal https://github.com/lightning/blips/pull/67

sample-lnd.conf

@@ -1440,8 +1440,8 @@
 ; Set to disable blinded route forwarding.
 ; protocol.no-route-blinding=false
 
-; Set to disable experimental endorsement signaling.
-; protocol.no-experimental-endorsement=false
+; Set to disable experimental accountability signaling.
+; protocol.no-experimental-accountability=false
 
 ; Set to enable support for RBF based coop close.
 ; protocol.rbf-coop-close=false