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>

tstest/integration/integration_test.go

@@ -14,7 +14,6 @@ import (
 	"flag"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"log"
 	"net/http"
 	"net/http/httptest"
@@ -653,7 +652,7 @@ func newTestNode(t *testing.T, env *testEnv) *testNode {
 func (n *testNode) diskPrefs() *ipn.Prefs {
 	t := n.env.t
 	t.Helper()
-	if _, err := ioutil.ReadFile(n.stateFile); err != nil {
+	if _, err := os.ReadFile(n.stateFile); err != nil {
 		t.Fatalf("reading prefs: %v", err)
 	}
 	fs, err := store.NewFileStore(nil, n.stateFile)
@@ -768,7 +767,7 @@ func (op *nodeOutputParser) parseLines() {
 	if len(buf) == 0 {
 		op.buf.Reset()
 	} else {
-		io.CopyN(ioutil.Discard, &op.buf, int64(op.buf.Len()-len(buf)))
+		io.CopyN(io.Discard, &op.buf, int64(op.buf.Len()-len(buf)))
 	}
 }