bump go to v1.21 #2196

Job	Run time
Unit Test (knative/serving)	9m 21s
Unit Test (knative/networking)	4m 56s
Unit Test (knative/eventing)	13m 20s
Unit Test (knative/operator)	6m 45s
Unit Test (knative/client)	7m 13s
Unit Test (knative-extensions/eventing-autoscaler-keda)	5m 17s
Unit Test (knative-extensions/eventing-ceph)	4m 41s
Unit Test (knative-extensions/eventing-github)	5m 31s
Unit Test (knative-extensions/eventing-gitlab)	4m 59s
Unit Test (knative-extensions/eventing-kafka)	7m 33s
Unit Test (knative-extensions/eventing-kafka-broker)	9m 54s
Unit Test (knative-extensions/eventing-rabbitmq)	11m 28s
Unit Test (knative-extensions/eventing-redis)	6m 21s
Unit Test (knative-extensions/kn-plugin-admin)	4m 33s
Unit Test (knative-extensions/net-certmanager)	4m 43s
Unit Test (knative-extensions/net-contour)	4m 49s
Unit Test (knative-extensions/net-http01)	4m 8s
Unit Test (knative-extensions/net-istio)	6m 44s
Unit Test (knative-extensions/net-kourier)	4m 36s
	2h 6m 52s

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