refactor: move from io/ioutil to io and os package (#1843)

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>

server/server_test.go

@@ -16,10 +16,11 @@ package server_test
 import (
 	"bytes"
 	"errors"
-	"io/ioutil"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
+	"os"
 	"testing"
 	"time"
 
@@ -36,7 +37,7 @@ import (
 
 func TestNewServer(t *testing.T) {
 	t.Log("Run through NewServer constructor")
-	tmpDir, err := ioutil.TempDir("", "")
+	tmpDir, err := os.MkdirTemp("", "")
 	Ok(t, err)
 	_, err = server.NewServer(server.UserConfig{
 		DataDir:     tmpDir,
@@ -48,7 +49,7 @@ func TestNewServer(t *testing.T) {
 // todo: test what happens if we set different flags. The generated config should be different.
 
 func TestNewServer_InvalidAtlantisURL(t *testing.T) {
-	tmpDir, err := ioutil.TempDir("", "")
+	tmpDir, err := os.MkdirTemp("", "")
 	Ok(t, err)
 	_, err = server.NewServer(server.UserConfig{
 		DataDir:     tmpDir,
@@ -138,7 +139,7 @@ func TestHealthz(t *testing.T) {
 	w := httptest.NewRecorder()
 	s.Healthz(w, req)
 	Equals(t, http.StatusOK, w.Result().StatusCode)
-	body, _ := ioutil.ReadAll(w.Result().Body)
+	body, _ := io.ReadAll(w.Result().Body)
 	Equals(t, "application/json", w.Result().Header["Content-Type"][0])
 	Equals(t,
 		`{