nrf_modem: Fix copyright notice in DECT header

This file is not confidental, replace the text about confidentiality
with a tag to the Nordic license.

Signed-off-by: Markus Rekdal <markus.rekdal@nordicsemi.no>

Author: nordic-mare

nrf_modem/include/nrf_modem_dect.h

@@ -1,9 +1,7 @@
 /*
- * Copyright (c) 2026 Nordic Semiconductor ASA. All Rights Reserved.
+ * Copyright (c) 2026 Nordic Semiconductor ASA
  *
- * The information contained herein is confidential property of Nordic Semiconductor ASA.
- * The use, copying, transfer or disclosure of such information is prohibited except by
- * express written agreement with Nordic Semiconductor ASA.
+ * SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
  */
 
 /**