🐛 Replace check for 'TYPO3_MODE' with check for 'TYPO3'

The TYPO3_MODE variable was deprecated with TYPO3 v11 and replaced
with the TYPO3 constant.

Refs: #2

ext_localconf.php

@@ -1,6 +1,6 @@
 <?php
 
-if (!defined('TYPO3_MODE')) {
+if (!defined('TYPO3')) {
     die('Access denied.');
 }