lnrpc: update grpc-gateway library to v2

lnrpc/routerrpc/router_server.go

@@ -12,7 +12,7 @@ import (
 
 	"github.com/btcsuite/btcd/wire"
 	"github.com/btcsuite/btcutil"
-	"github.com/grpc-ecosystem/grpc-gateway/runtime"
+	"github.com/grpc-ecosystem/grpc-gateway/v2/runtime"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/lnrpc"
 	"github.com/lightningnetwork/lnd/lntypes"
@@ -147,6 +147,11 @@ type Server struct {
 	shutdown                 int32 // To be used atomically.
 	forwardInterceptorActive int32 // To be used atomically.
 
+	// Required by the grpc-gateway/v2 library for forward compatibility.
+	// Must be after the atomically used variables to not break struct
+	// alignment.
+	UnimplementedRouterServer
+
 	cfg *Config
 
 	quit chan struct{}