Update net-kourier nightly (#11804)
Update net-kourier nightly (#11804)

bumping knative.dev/net-kourier 587718a...b6929c8:
  > b6929c8 Bump Golang version to 1.16 (# 591)

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