MAINTAINERS: ena: update AMAZON ETHERNET maintainers

Remove Shay Agroskin and Saeed Bishara.
Promote David Arinzon to maintainer.

Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Link: https://patch.msgid.link/20260301191652.5916-1-akiyano@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>