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>

examples/example-app/main.go

@@ -8,7 +8,6 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"io/ioutil"
 	"log"
 	"net"
 	"net/http"
@@ -43,7 +42,7 @@ type app struct {
 // return an HTTP client which trusts the provided root CAs.
 func httpClientForRootCAs(rootCAs string) (*http.Client, error) {
 	tlsConfig := tls.Config{RootCAs: x509.NewCertPool()}
-	rootCABytes, err := ioutil.ReadFile(rootCAs)
+	rootCABytes, err := os.ReadFile(rootCAs)
 	if err != nil {
 		return nil, fmt.Errorf("failed to read root-ca: %v", err)
 	}