MAINTAINERS: update TUN/TAP maintainers

MAINTAINERS: update TUN/TAP maintainers

Willem and Jason have agreed to take over the maintainer
duties for TUN/TAP, thank you!

There's an existing entry for TUN/TAP which only covers
the user mode Linux implementation.
Since we haven't heard from Maxim on the list for almost
a decade, extend that entry and take it over, rather than
adding a new one.

Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20230802182843.4193099-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>