Handle ioutil deprecations (#53526)

* replace ioutil.ReadFile -> os.ReadFile

* replace ioutil.ReadAll -> io.ReadAll

* replace ioutil.TempFile -> os.CreateTemp

* replace ioutil.NopCloser -> io.NopCloser

* replace ioutil.WriteFile -> os.WriteFile

* replace ioutil.TempDir -> os.MkdirTemp

* replace ioutil.Discard -> io.Discard

pkg/tests/api/alerting/api_alertmanager_configuration_test.go

@@ -3,7 +3,7 @@ package alerting
 import (
 	"encoding/json"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"net/http"
 	"regexp"
 	"testing"
@@ -87,7 +87,7 @@ func TestAlertmanagerConfigurationIsTransactional(t *testing.T) {
 }
 `
 		resp := postRequest(t, alertConfigURL, payload, http.StatusBadRequest) // nolint
-		b, err := ioutil.ReadAll(resp.Body)
+		b, err := io.ReadAll(resp.Body)
 		require.NoError(t, err)
 		var res map[string]interface{}
 		require.NoError(t, json.Unmarshal(b, &res))