Properly handle MySQL error code 4031 from PHP 8.4 (doctrine#6363)

|      Q       |   A
|------------- | -----------
| Type         | improvement
| Fixed issues | Prevents future issues ;-)

#### Summary

PHP 8.4 will support a new error code 4031 in mysqlnd when the
connection is dropped due to timeouts.

It has been introduced in:

mysql/mysql-server@14508bb

And PHP 8.4 will support it:
php/php-src#13618

The PR gets the test suite green again (mysqli + pdo_mysql). I have used
4.0.x as base, but feel free to change according to your preferences.

src/Driver/API/MySQL/ExceptionConverter.php

@@ -101,6 +101,7 @@ public function convert(Exception $exception, ?Query $query): DriverException
                 return new ConnectionException($exception, $query);
 
             case 2006:
+            case 4031:
                 return new ConnectionLost($exception, $query);
 
             case 1048: