doc: release-notes-3.7: Document removal of old gsm modules

The GSM_PPP device driver and its direct dependencies, UART_MUX
and GSM_MUX have been removed. The net/gsm_modem sample has been
removed and the net/cloud/tagoio and net/mgmt/updatehub samples
have been updated to align with this change.

This commit documents all above.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>