migration symfony 5 4 (#300)

* symfony 5.4 (diff dev)

* symfony 5.4 (working)

* symfony 5.4 (update autoload)

* symfony 5.4 (remove swiftmailer mailer implementation)

* symfony 5.4 (php doc and split Global accessor class)


### Impacted packages:

composer require php:">=7.2.5 <8.0.0" symfony/console:5.4.* symfony/dotenv:5.4.* symfony/framework-bundle:5.4.* symfony/twig-bundle:5.4.* symfony/yaml:5.4.* --update-with-dependencies

composer require symfony/stopwatch:5.4.* symfony/web-profiler-bundle:5.4.* --dev --update-with-dependencies

lib/symfony/polyfill-php81/bootstrap.php

@@ -0,0 +1,28 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+use Symfony\Polyfill\Php81 as p;
+
+if (\PHP_VERSION_ID >= 80100) {
+    return;
+}
+
+if (defined('MYSQLI_REFRESH_SLAVE') && !defined('MYSQLI_REFRESH_REPLICA')) {
+    define('MYSQLI_REFRESH_REPLICA', 64);
+}
+
+if (!function_exists('array_is_list')) {
+    function array_is_list(array $array): bool { return p\Php81::array_is_list($array); }
+}
+
+if (!function_exists('enum_exists')) {
+    function enum_exists(string $enum, bool $autoload = true): bool { return $autoload && class_exists($enum) && false; }
+}