Replace ioutil.Discard with io.Discard (#20707)

api4/apitestlib.go

@@ -381,7 +381,7 @@ func (th *TestHelper) TearDown() {
 
 func closeBody(r *http.Response) {
 	if r.Body != nil {
-		_, _ = io.Copy(ioutil.Discard, r.Body)
+		_, _ = io.Copy(io.Discard, r.Body)
 		_ = r.Body.Close()
 	}
 }