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

feature/default_sets.go

@@ -100,7 +100,7 @@ var defaultSetDesc = setDesc{
 		SetInit:    {}, // I
 		SetNodeAnn: {}, // N
 	},
-	lnwire.ExperimentalEndorsementOptional: {
+	lnwire.ExperimentalAccountabilityOptional: {
 		SetNodeAnn: {}, // N
 	},
 	lnwire.RbfCoopCloseOptionalStaging: {