gluon-web-admin: migrate to spdx license header

package/gluon-web-admin/files/lib/gluon/config-mode/view/admin/upgrade_confirm.html

 <%#
-Copyright 2008 Steven Barth <steven@midlink.org>
-Copyright 2008-2009 Jo-Philipp Wich <xm@subsignal.org>
-
-Licensed under the Apache License, Version 2.0 (the "License");
-you may not use this file except in compliance with the License.
-You may obtain a copy of the License at
-
-	http://www.apache.org/licenses/LICENSE-2.0
+SPDX-FileCopyrightText: 2008 Steven Barth <steven@midlink.org>
+SPDX-FileCopyrightText: 2008-2009 Jo-Philipp Wich <xm@subsignal.org>
+SPDX-License-Identifier: Apache-2.0
 -%>
 
 <h2><%:Upgrade firmware%></h2>