Don't truncate to 64k in mb_str_replace

src/PhpSpreadsheet/Calculation/Functions.php

@@ -703,7 +703,7 @@ if ((!function_exists('mb_str_replace')) &&
             $r = !is_array($replace) ? $replace : (array_key_exists($key, $replace) ? $replace[$key] : '');
             $pos = mb_strpos($subject, $s, 0, 'UTF-8');
             while ($pos !== false) {
-                $subject = mb_substr($subject, 0, $pos, 'UTF-8') . $r . mb_substr($subject, $pos + mb_strlen($s, 'UTF-8'), 65535, 'UTF-8');
+                $subject = mb_substr($subject, 0, $pos, 'UTF-8') . $r . mb_substr($subject, $pos + mb_strlen($s, 'UTF-8'), null, 'UTF-8');
                 $pos = mb_strpos($subject, $s, $pos + mb_strlen($r, 'UTF-8'), 'UTF-8');
             }
         }