fix: fix linter error

backend/billing/client.go

 package billing
 
 import (
-	"io/ioutil"
+	"io"
 	"strconv"
 	"time"
 
@@ -52,7 +52,7 @@ func PullCDRFile(c *ftp.ServerConn, fileName string) ([]byte, error) {
 		return nil, err
 	}
 
-	cdr, err1 := ioutil.ReadAll(r)
+	cdr, err1 := io.ReadAll(r)
 
 	return cdr, err1
 }

backend/billing/server.go

@@ -3,7 +3,6 @@ package billing
 
 import (
 	"encoding/json"
-	"io/ioutil"
 	"os"
 	"strconv"
 	"sync"
@@ -79,7 +78,7 @@ func OpenServer(wg *sync.WaitGroup) *BillingDomain {
 		return nil
 	}
 
-	if err := ioutil.WriteFile(confFile, file, 0o600); err != nil { //nolint: gomnd
+	if err := os.WriteFile(confFile, file, 0o600); err != nil { //nolint: gomnd
 		logger.BillingLog.Errorf("Couldn't create conf file %v", confFile)
 		return nil
 	}

backend/factory/factory.go

@@ -6,7 +6,7 @@ package factory
 
 import (
 	"fmt"
-	"io/ioutil"
+	"os"
 
 	"github.com/asaskevich/govalidator"
 	"gopkg.in/yaml.v2"
@@ -22,7 +22,7 @@ func InitConfigFactory(f string, cfg *Config) error {
 		// Use default config path
 		f = WebuiDefaultConfigPath
 	}
-	if content, err := ioutil.ReadFile(f); err != nil {
+	if content, err := os.ReadFile(f); err != nil {
 		return fmt.Errorf("[Factory] %+v", err)
 	} else {
 		logger.CfgLog.Infof("Read config from [%s]", f)

backend/webui_service/webui_init.go

 package webui_service
 
 import (
-	"io/ioutil"
+	"io"
 	"os"
 	"os/signal"
 	"runtime/debug"
@@ -39,14 +39,14 @@ func (a *WebuiApp) SetLogEnable(enable bool) {
 	logger.MainLog.Infof("Log enable is set to [%v]", enable)
 	if enable && logger.Log.Out == os.Stderr {
 		return
-	} else if !enable && logger.Log.Out == ioutil.Discard {
+	} else if !enable && logger.Log.Out == io.Discard {
 		return
 	}
 	a.cfg.SetLogEnable(enable)
 	if enable {
 		logger.Log.SetOutput(os.Stderr)
 	} else {
-		logger.Log.SetOutput(ioutil.Discard)
+		logger.Log.SetOutput(io.Discard)
 	}
 }