lnrpc: update grpc-gateway library to v2

lnrpc/walletrpc/walletkit_server.go

@@ -22,7 +22,7 @@ import (
 	"github.com/btcsuite/btcutil/psbt"
 	"github.com/btcsuite/btcwallet/waddrmgr"
 	"github.com/btcsuite/btcwallet/wtxmgr"
-	"github.com/grpc-ecosystem/grpc-gateway/runtime"
+	"github.com/grpc-ecosystem/grpc-gateway/v2/runtime"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/labels"
@@ -179,6 +179,9 @@ type ServerShell struct {
 // to execute common wallet operations. This includes requesting new addresses,
 // keys (for contracts!), and publishing transactions.
 type WalletKit struct {
+	// Required by the grpc-gateway/v2 library for forward compatibility.
+	UnimplementedWalletKitServer
+
 	cfg *Config
 }