Add copyright header to docs/maintenance-synapse.md

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

docs/maintenance-synapse.md

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+SPDX-FileCopyrightText: 2019 - 2024 Slavi Pantaleev
+SPDX-FileCopyrightText: 2020 Marcel Partap
+SPDX-FileCopyrightText: 2021 - 2024 MDAD project contributors
+SPDX-FileCopyrightText: 2021 Aaron Raimist
+SPDX-FileCopyrightText: 2022 Dennis Ciba
+SPDX-FileCopyrightText: 2024 - 2025 Suguru Hirahara
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
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 # Synapse maintenance
 
 This document shows you how to perform various maintenance tasks related to the Synapse chat server.