minor #17934 [Mailer] Switching to new config format (ThomasLandauer)

This PR was squashed before being merged into the 5.4 branch.

Discussion
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[Mailer] Switching to new config format

Page: https://symfony.com/doc/5.4/mailer.html

Commits
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773cb0f [Mailer] Switching to new config format

Author: javiereguiluz

mailer.rst

@@ -55,14 +55,10 @@ over SMTP by configuring the DSN in your ``.env`` file (the ``user``,
 
         // config/packages/mailer.php
         use function Symfony\Component\DependencyInjection\Loader\Configurator\env;
-        use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;
-
-        return static function (ContainerConfigurator $container): void {
-            $container->extension('framework', [
-                'mailer' => [
-                    'dsn' => env('MAILER_DSN'),
-                ],
-            ]);
+        use Symfony\Config\FrameworkConfig;
+
+        return static function (FrameworkConfig $framework): void {
+            $framework->mailer()->dsn(env('MAILER_DSN'));
         };
 
 .. caution::