Remove deprecated io/ioutil (#15707)

docs/iam/access-manager-plugin.go

@@ -23,7 +23,7 @@ package main
 import (
 	"encoding/json"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"log"
 	"net/http"
 	"strings"
@@ -41,7 +41,7 @@ type Result struct {
 }
 
 func mainHandler(w http.ResponseWriter, r *http.Request) {
-	body, err := ioutil.ReadAll(r.Body)
+	body, err := io.ReadAll(r.Body)
 	if err != nil {
 		writeErrorResponse(w, err)
 		return