Update docs/maintenance-postgres.md: add a copyright header

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

docs/maintenance-postgres.md

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+SPDX-FileCopyrightText: 2019 - 2024 Slavi Pantaleev
+SPDX-FileCopyrightText: 2020 - 2021 Aaron Raimist
+SPDX-FileCopyrightText: 2020 Hardy Erlinger
+SPDX-FileCopyrightText: 2021 - 2024 MDAD project contributors
+SPDX-FileCopyrightText: 2021 Marc Leuser
+SPDX-FileCopyrightText: 2024 Kim Brose
+SPDX-FileCopyrightText: 2024 Suguru Hirahara
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 # PostgreSQL maintenance
 
 This document shows you how to perform various maintenance tasks related to the Postgres database server used by Matrix.