doc: release: 2.7: Add note about modem
doc: release: 2.7: Add note about modem

Added modem gsm_ppp changes.

Signed-off-by: Gerson Fernando Budke <0ad1c92223f8f375c46e39c6141096f3d30896b4@gmail.com>

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