po: Update Dutch header boilerplate with an actual copyright line

This caused the author checks to fail, and consequently the CI pipelines
too. Unfortunately the release script did not catch this, probably due
to not properly installing the build dependencies.

Warned-by: i18nspector
(cherry picked from commit a436a216)

scripts/po/nl.po

 # Translation of dpkg-dev into Dutch.
-# Copyright (C) YEAR Dpkg Developers
+# Copyright © 1994-2023 Dpkg Developers
 # This file is distributed under the same license as the dpkg package.
 # Frans Spiesschaert <Frans.Spiesschaert@yucom.be>, 2023.
 #