Add copyright header to docs/importing-postgres.md

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

docs/importing-postgres.md

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+SPDX-FileCopyrightText: 2019 - 2023 Slavi Pantaleev
+SPDX-FileCopyrightText: 2020 - 2021 MDAD project contributors
+SPDX-FileCopyrightText: 2021 Kim Brose
+SPDX-FileCopyrightText: 2024 - 2025 Suguru Hirahara
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+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
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 # Importing an existing Postgres database from another installation (optional)
 
 You can manually import your database from a previous default installation of Synapse.