MAINTAINERS: Update Kuniyuki Iwashima's email address.

I left Amazon and joined Google, so let's map the email
addresses accordingly.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250610235734.88540-1-kuniyu@google.com


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