Replace ioutil.Discard with io.Discard

Address a report by staticcheck: "io/ioutil" has been deprecated since
Go 1.16: As of Go 1.16, the same functionality is now provided by
package io or package os, and those implementations should be preferred
in new code. […] (SA1019)

Author: hansmi

collector_test.go

@@ -4,7 +4,7 @@ import (
 	"bytes"
 	"context"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"log"
 	"testing"
 
@@ -20,7 +20,7 @@ func disableLogOutput(t *testing.T) {
 
 	previous := log.Writer()
 
-	log.SetOutput(ioutil.Discard)
+	log.SetOutput(io.Discard)
 
 	t.Cleanup(func() {
 		log.SetOutput(previous)