[Site] upgrade to Symfony 6.4 stable

Author: kbond

Diff for: ux.symfony.com/assets/controllers.json

@@ -6,6 +6,7 @@
                 "fetch": "lazy",
                 "autoimport": {
                     "tom-select/dist/css/tom-select.default.css": false,
+                    "tom-select/dist/css/tom-select.bootstrap4.css": false,
                     "tom-select/dist/css/tom-select.bootstrap5.css": true
                 }
             }

Diff for: ux.symfony.com/composer.json

@@ -29,6 +29,7 @@
         "symfony/notifier": "6.4.*",
         "symfony/proxy-manager-bridge": "6.4.*",
         "symfony/runtime": "6.4.*",
+        "symfony/serializer": "6.4.*",
         "symfony/stimulus-bundle": "2.x-dev",
         "symfony/translation": "6.4.*",
         "symfony/twig-bundle": "6.4.*",
@@ -91,7 +92,8 @@
     "scripts": {
         "auto-scripts": {
             "cache:clear": "symfony-cmd",
-            "assets:install %PUBLIC_DIR%": "symfony-cmd"
+            "assets:install %PUBLIC_DIR%": "symfony-cmd",
+            "importmap:install": "symfony-cmd"
         },
         "post-install-cmd": [
             "@auto-scripts"