hack/boilerplate: update go boilerplate

Author: zchee

hack/boilerplate/boilerplate.go.txt

@@ -1,4 +1,3 @@
-// Copyright YEAR The Go Language Server Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
+// Copyright YEAR The Go Language Server Authors
+// SPDX-License-Identifier: BSD-3-Clause