contractcourt+lnrpc+lnd: s/BreachArbiter/BreachArbitrator

Since we have two other examples of XArbitrator, we rename
BreachArbiter to BreachArbitrator to keep things consistent.
The aim is to reduce the amount of lore you need to know to
intuit where things are or what they do.

lnrpc/walletrpc/walletkit_server.go

@@ -201,7 +201,7 @@ var (
 	// are numbered differently. The protobuf enum cannot be renumbered
 	// because this would break backwards compatibility with older clients,
 	// and the native enum cannot be renumbered because it is stored in the
-	// watchtower and breacharbiter databases.
+	// watchtower and BreachArbitrator databases.
 	//
 	//nolint:lll
 	allWitnessTypes = map[input.WitnessType]WitnessType{