refactor: move from io/ioutil to io and os package

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>

storage/conformance/gen_jwks.go

@@ -12,8 +12,8 @@ import (
 	"encoding/json"
 	"go/format"
 	"io"
-	"io/ioutil"
 	"log"
+	"os"
 	"text/template"
 
 	jose "gopkg.in/square/go-jose.v2"
@@ -104,7 +104,7 @@ func main() {
 	if err != nil {
 		log.Fatalf("gofmt failed: %v", err)
 	}
-	if err := ioutil.WriteFile("jwks.go", out, 0644); err != nil {
+	if err := os.WriteFile("jwks.go", out, 0644); err != nil {
 		log.Fatal(err)
 	}
 }