MAINTAINERS: update Matthieu's email address

Use my kernel.org account instead.

The other one will bounce by the end of the year.

Signed-off-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231004-send-net-20231004-v1-3-28de4ac663ae@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>