MAINTAINERS: Update email address for Quentin Monnet

With Isovalent being acquired by Cisco, I expect my related email
address to disappear sooner or later. Update my email entries in
MAINTAINERS and .mailmap with my kernel.org address instead.

Signed-off-by: Quentin Monnet <qmo@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/lkml/20240315133606.65971-1-qmo@kernel.org