all: Remove usage of deprecated io/ioutil (#7971)

As of Go 1.16 io/ioutil is deprecated. This replaces usage with the
corresponding functions in package os and package io.

cmd/syncthing/cli/utils.go

@@ -10,7 +10,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"mime"
 	"net/http"
 	"os"
@@ -23,7 +23,7 @@ import (
 )
 
 func responseToBArray(response *http.Response) ([]byte, error) {
-	bytes, err := ioutil.ReadAll(response.Body)
+	bytes, err := io.ReadAll(response.Body)
 	if err != nil {
 		return nil, err
 	}