Add copyright header to docs/maintenance-migrating.md

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/maintenance-migrating.md

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+SPDX-FileCopyrightText: 2019 - 2024 Slavi Pantaleev
+SPDX-FileCopyrightText: 2019 Michael Haak
+SPDX-FileCopyrightText: 2021 - 2023 MDAD project contributors
+SPDX-FileCopyrightText: 2024 - 2025 Suguru Hirahara
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+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
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 # Migrating to new server
 
 This documentation explains how to migrate your Matrix services (server, client, bridges, etc.) and data **from an old server to a new server**.