Update net-kourier nightly (#12916)
Update net-kourier nightly (#12916)

bumping knative.dev/net-kourier 475b0cd...18dd5c2:
  > 18dd5c2 Update community files (# 836)
  > 31f2405 Bump envoy version (# 827)

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