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/dependency-injection/Extension/ExtensionInterface.php

@@ -30,7 +30,7 @@ interface ExtensionInterface
     /**
      * Returns the namespace to be used for this extension (XML namespace).
      *
-     * @return string The XML namespace
+     * @return string
      */
     public function getNamespace();
 
@@ -46,7 +46,7 @@ interface ExtensionInterface
      *
      * This alias is also the mandatory prefix to use when using YAML.
      *
-     * @return string The alias
+     * @return string
      */
     public function getAlias();
 }