Update net-istio nightly (#10964)
Update net-istio nightly (#10964)

bumping knative.dev/net-istio 2308c2f...3a9a02e:
  > 3a9a02e Add configuration for graceful shutdown to ingressgateways (# 553)

Signed-off-by: Knative Automation <f415700d4d0ce1c81810fe7553e3eea2bed16231@knative.team>
knative/serving,knative/serving