lnrpc: update grpc-gateway library to v2

lnrpc/autopilotrpc/autopilot_server.go

@@ -8,7 +8,7 @@ import (
 	"sync/atomic"
 
 	"github.com/btcsuite/btcd/btcec"
-	"github.com/grpc-ecosystem/grpc-gateway/runtime"
+	"github.com/grpc-ecosystem/grpc-gateway/v2/runtime"
 	"github.com/lightningnetwork/lnd/autopilot"
 	"github.com/lightningnetwork/lnd/lnrpc"
 	"google.golang.org/grpc"
@@ -65,6 +65,11 @@ type Server struct {
 	started  int32 // To be used atomically.
 	shutdown 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.
+	UnimplementedAutopilotServer
+
 	cfg *Config
 
 	manager *autopilot.Manager