Remove `ioutil` (#18222)

- Don't use `ioutil` package anymore as it doesn't anything special
anymore since Go 1.16:

```
// As of Go 1.16, the same functionality is now provided
// by package io or package os, and those implementations
// should be preferred in new code.
```

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

models/asymkey/ssh_key_test.go

@@ -7,7 +7,7 @@ package asymkey
 
 import (
 	"bytes"
-	"io/ioutil"
+	"os"
 	"os/exec"
 	"path/filepath"
 	"strings"
@@ -325,7 +325,7 @@ func TestFromOpenSSH(t *testing.T) {
 			sigPath := dataPath + ".sig"
 			run(t, nil, "ssh-keygen", "-Y", "sign", "-n", "file", "-f", privPath, dataPath)
 
-			sigBytes, err := ioutil.ReadFile(sigPath)
+			sigBytes, err := os.ReadFile(sigPath)
 			if err != nil {
 				t.Fatal(err)
 			}
@@ -467,7 +467,7 @@ func TestRoundTrip(t *testing.T) {
 
 func write(t *testing.T, d []byte, fp ...string) string {
 	p := filepath.Join(fp...)
-	if err := ioutil.WriteFile(p, d, 0o600); err != nil {
+	if err := os.WriteFile(p, d, 0o600); err != nil {
 		t.Fatal(err)
 	}
 	return p

models/migrations/migrations_test.go

@@ -8,7 +8,6 @@ import (
 	"context"
 	"database/sql"
 	"fmt"
-	"io/ioutil"
 	"os"
 	"path"
 	"path/filepath"
@@ -58,7 +57,7 @@ func TestMain(m *testing.M) {
 		setting.CustomConf = giteaConf
 	}
 
-	tmpDataPath, err := ioutil.TempDir("", "data")
+	tmpDataPath, err := os.MkdirTemp("", "data")
 	if err != nil {
 		fmt.Printf("Unable to create temporary data path %v\n", err)
 		os.Exit(1)

modules/updatechecker/update_checker.go

@@ -5,7 +5,7 @@
 package updatechecker
 
 import (
-	"io/ioutil"
+	"io"
 	"net/http"
 
 	"code.gitea.io/gitea/modules/appstate"
@@ -43,7 +43,7 @@ func GiteaUpdateChecker(httpEndpoint string) error {
 		return err
 	}
 	defer resp.Body.Close()
-	body, err := ioutil.ReadAll(resp.Body)
+	body, err := io.ReadAll(resp.Body)
 	if err != nil {
 		return err
 	}