Update module cenkalti/backoff to v4

Author: renovate-bot

go.mod

@@ -8,7 +8,7 @@ require (
 	github.com/ajg/form v1.5.1 // indirect
 	github.com/aws/aws-lambda-go v1.11.1
 	github.com/aws/aws-sdk-go v1.28.5
-	github.com/cenkalti/backoff v2.2.1+incompatible
+	github.com/cenkalti/backoff/v4 v4.0.2
 	github.com/certifi/gocertifi v0.0.0-20190506164543-d2eda7129713 // indirect
 	github.com/docker/distribution v2.7.1+incompatible // indirect
 	github.com/docker/docker v1.13.1 // indirect