release note/migration guide: Mention fuzzing changes

The fuzzing support only worked when targetting the
native_posix board, and in a way which was very coupled
with the fuzzing sample.
This has now been refactored, so those who may have
relied on hooking to the board in the same way as this
sample will need to adapt.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>