doc: releases: add release note for OpenThread update

Add an entry to the release note and migration guide regarding
moving the OpenThread implementation from the L2 networking layer
to the OpenThread module.

The migration guide note contains information about deprecated
functions, explains what functions should be used instead and
provides a guide for required updates in users' applications.

Signed-off-by: Arkadiusz Balys <arkadiusz.balys@nordicsemi.no>