Fixing all deprecated use of sets.String #2208

Job	Run time
Unit Test (knative/serving)	5m 58s
Unit Test (knative/networking)	4m 55s
Unit Test (knative/eventing)	12m 55s
Unit Test (knative/operator)	6m 50s
Unit Test (knative/client)	7m 23s
Unit Test (knative-extensions/eventing-autoscaler-keda)	5m 15s
Unit Test (knative-extensions/eventing-ceph)	4m 56s
Unit Test (knative-extensions/eventing-github)	5m 40s
Unit Test (knative-extensions/eventing-gitlab)	5m 3s
Unit Test (knative-extensions/eventing-kafka)	6m 46s
Unit Test (knative-extensions/eventing-kafka-broker)	9m 33s
Unit Test (knative-extensions/eventing-rabbitmq)	11m 21s
Unit Test (knative-extensions/eventing-redis)	6m 2s
Unit Test (knative-extensions/kn-plugin-admin)	4m 28s
Unit Test (knative-extensions/net-certmanager)	4m 26s
Unit Test (knative-extensions/net-contour)	4m 48s
Unit Test (knative-extensions/net-http01)	4m 2s
Unit Test (knative-extensions/net-istio)	6m 18s
Unit Test (knative-extensions/net-kourier)	3m 14s
	1h 59m 53s

Provide feedback