Bluetooth: Host: Update API `bt_gatt_notify_multiple`
Bluetooth: Host: Update API `bt_gatt_notify_multiple`

This API shall now be a more low-level API that sends a single PDU.

Signed-off-by: Aleksander Wasaznik <c0d989573cb73e7c43339e7aaab9bc0cb8ebe8c5@nordicsemi.no>

zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr