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>

test/benchmark/picasso.go

@@ -8,7 +8,6 @@ import (
 	"flag"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"os"
 	"path/filepath"
 	"strings"
@@ -163,7 +162,7 @@ func generateChart(title string, file string, format chart.RendererProvider, ser
 		return err
 	}
 
-	err = ioutil.WriteFile(file, buffer.Bytes(), 0644)
+	err = os.WriteFile(file, buffer.Bytes(), 0644)
 	if err != nil {
 		return err
 	}