[Automated] Update backstage-plugins-eventing nightly

[knative-automation]

/assign @knative-extensions/backstage-plugins-writers
/cc @knative-extensions/backstage-plugins-writers

Produced via:

for x in eventing-crds.yaml eventing-core.yaml in-memory-channel.yaml mt-channel-broker.yaml; do
  curl https://storage.googleapis.com/knative-nightly/eventing/latest/$x > ${GITHUB_WORKSPACE}/./third_party/eventing-latest/$x
done

Details:

bumping knative.dev/eventing 71a6c9f...e169933:
  > e169933 Run eventing office hours Slack reminder every other week (# 8855)

[knative-prow]

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