*: move from io/ioutil to io and os packages

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>

client/pkg/fileutil/read_dir_test.go

@@ -15,7 +15,6 @@
 package fileutil
 
 import (
-	"io/ioutil"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -23,10 +22,10 @@ import (
 )
 
 func TestReadDir(t *testing.T) {
-	tmpdir, err := ioutil.TempDir("", "")
+	tmpdir, err := os.MkdirTemp("", "")
 	defer os.RemoveAll(tmpdir)
 	if err != nil {
-		t.Fatalf("unexpected ioutil.TempDir error: %v", err)
+		t.Fatalf("unexpected os.MkdirTemp error: %v", err)
 	}
 
 	files := []string{"def", "abc", "xyz", "ghi"}