drivers: modem: deprecate MODEM_QUECTEL_BG9X

The modem_cellular implementation should suffice to handle the base
usage of generic BG9x family devices, so deprecate the quectel-bg9x.c
implementation.

Signed-off-by: Guilherme Costa <guilhermecosta@stratioautomotive.com>

Author: guildeol-stra

drivers/modem/Kconfig.quectel-bg9x

@@ -4,12 +4,13 @@
 # SPDX-License-Identifier: Apache-2.0
 
 config MODEM_QUECTEL_BG9X
-	bool "Quectel modem driver"
+	bool "Quectel modem driver [ DEPRECATED ]"
 	select MODEM_CONTEXT
 	select MODEM_CMD_HANDLER
 	select MODEM_IFACE_UART
 	select MODEM_SOCKET
 	select NET_SOCKETS_OFFLOAD
+	select DEPRECATED
 	help
 	  Choose this setting to enable quectel BG9x LTE-CatM1/NB-IoT modem
 	  driver.