Update the copyright header template

This updates the text suggested by our copyright header lint rule.

.eslintrc.cjs

@@ -1,17 +1,8 @@
 const COPYRIGHT_HEADER = `/*
-Copyright %%CURRENT_YEAR%% New Vector Ltd
+Copyright %%CURRENT_YEAR%% New Vector Ltd.
 
-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
-
-Unless required by applicable law or agreed to in writing, software
-distributed under the License is distributed on an "AS IS" BASIS,
-WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-See the License for the specific language governing permissions and
-limitations under the License.
+SPDX-License-Identifier: AGPL-3.0-only
+Please see LICENSE in the repository root for full details.
 */
 
 `;