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

The io/ioutil package has been deprecated as of Go 1.16 [1]. This commit
replaces the existing io/ioutil functions with their new definitions in
io and os packages.

Reference: https://golang.org/doc/go1.16#ioutil
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

net/dnsfallback/update-dns-fallbacks.go

@@ -10,7 +10,6 @@ package main
 import (
 	"encoding/json"
 	"fmt"
-	"io/ioutil"
 	"log"
 	"net/http"
 	"os"
@@ -42,7 +41,7 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
-	if err := ioutil.WriteFile("dns-fallback-servers.json", out, 0644); err != nil {
+	if err := os.WriteFile("dns-fallback-servers.json", out, 0644); err != nil {
 		log.Fatal(err)
 	}
 }