CREDITS: Update email address for Mat Martineau

Changing my email address in CREDITS to be consistent with what's in use
in MAINTAINERS and mailmap. Also removed extra date information from the
CREDITS entry since I'm a maintainer for MPTCP again.

Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20230502000545.2899055-1-martineau@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>