MAINTAINERS: Update MPTCP maintainer list and CREDITS

My responsibilities at Intel have changed, so I'm handing off exclusive
MPTCP subsystem maintainer duties to Matthieu. It has been a privilege
to see MPTCP through its initial upstreaming and first few years in the
upstream kernel!

Acked-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Link: https://lore.kernel.org/r/20230120231121.36121-1-mathew.j.martineau@linux.intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>