refactor: move from io/ioutil to io and os package (#2236)

The io/ioutil package has been deprecated as of Go 1.16, see
https://golang.org/doc/go1.16#ioutil. This commit replaces the existing
io/ioutil functions with their new definitions in io and os packages.

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

pkg/controller/api_test.go

@@ -20,7 +20,7 @@ import (
 	"context"
 	"flag"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"log"
 	"net/http"
 	"os"
@@ -396,7 +396,7 @@ func TestMain(m *testing.M) {
 	}
 	assert(found, "incorrect response content type")
 
-	_, err = ioutil.ReadAll(resp.Body)
+	_, err = io.ReadAll(resp.Body)
 	panicIf(err)
 
 	os.Exit(m.Run())