Correctly mention previous copyright (#16820)

During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.

tests/storage/test_database.py

@@ -1,6 +1,7 @@
 #
 # This file is licensed under the Affero General Public License (AGPL) version 3.
 #
+# Copyright 2020 The Matrix.org Foundation C.I.C.
 # Copyright (C) 2023 New Vector, Ltd
 #
 # This program is free software: you can redistribute it and/or modify