gluon-web: migrate to spdx license header

package/gluon-web/files/lib/gluon/web/view/error/404.html

 <%#
- Copyright 2008 Steven Barth <steven@midlink.org>
- Copyright 2008 Jo-Philipp Wich <jow@openwrt.org>
- Licensed to the public under the Apache License 2.0.
+SPDX-FileCopyrightText: 2008 Steven Barth <steven@midlink.org>
+SPDX-FileCopyrightText: 2008 Jo-Philipp Wich <jow@openwrt.org>
+SPDX-License-Identifier: Apache-2.0
 -%>
 
 <h2 name="content">404 <%:Not Found%></h2>

package/gluon-web/files/lib/gluon/web/view/error/500.html

 <%#
- Copyright 2008 Steven Barth <steven@midlink.org>
- Copyright 2008 Jo-Philipp Wich <jow@openwrt.org>
- Licensed to the public under the Apache License 2.0.
+SPDX-FileCopyrightText: 2008 Steven Barth <steven@midlink.org>
+SPDX-FileCopyrightText: 2008 Jo-Philipp Wich <jow@openwrt.org>
+SPDX-License-Identifier: Apache-2.0
 -%>
 
 <h2 name="content">500 <%:Internal Server Error%></h2>