change random string library because of deprecated code in old one

src/Application/Model/d3RandomGenerator.php

@@ -0,0 +1,27 @@
+<?php
+
+
+namespace D3\Totp\Application\Model;
+
+use Zend\Math\Rand;
+
+class d3RandomGenerator extends Rand
+{
+    const CHAR_UPPER     = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
+    const CHAR_LOWER     = 'abcdefghijklmnopqrstuvwxyz';
+    const CHAR_DIGITS    = '0123456789';
+    const CHAR_UPPER_HEX = 'ABCDEF';
+    const CHAR_LOWER_HEX = 'abcdef';
+    const CHAR_BASE64    = '+/';
+    const CHAR_SYMBOLS   = '!"#$%&\'()* +,-./:;<=>?@[\]^_`{|}~';
+    const CHAR_BRACKETS  = '()[]{}<>';
+    const CHAR_PUNCT     = ',.;:';
+
+    /**
+     * @return string
+     */
+    public static function getRandomTotpBackupCode()
+    {
+        return self::getString(6, self::CHAR_DIGITS);
+    }
+}