Bump google.golang.org/protobuf from 1.30.0 to 1.31.0 #1969

Job	Run time
Unit Test (knative/serving)	17m 30s
Unit Test (knative/networking)	7m 32s
Unit Test (knative/eventing)	26m 9s
Unit Test (knative/operator)	11m 18s
Unit Test (knative/client)	13m 3s
Unit Test (knative-sandbox/eventing-autoscaler-keda)	9m 1s
Unit Test (knative-sandbox/eventing-ceph)	9m 6s
Unit Test (knative-sandbox/eventing-github)	9m 51s
Unit Test (knative-sandbox/eventing-gitlab)	9m 2s
Unit Test (knative-sandbox/eventing-kafka)	13m 6s
Unit Test (knative-sandbox/eventing-kafka-broker)	14m 53s
Unit Test (knative-sandbox/eventing-rabbitmq)	16m 17s
Unit Test (knative-sandbox/eventing-redis)	10m 31s
Unit Test (knative-sandbox/kn-plugin-admin)	8m 58s
Unit Test (knative-sandbox/net-certmanager)	8m 59s
Unit Test (knative-sandbox/net-contour)	8m 28s
Unit Test (knative-sandbox/net-http01)	7m 36s
Unit Test (knative-sandbox/net-istio)	9m 17s
Unit Test (knative-sandbox/net-kourier)	9m 10s
	3h 39m 47s

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