[Demo] Update errors route config for Symfony 8

Changed from `.xml` to `.php` as XML routing config was removed in Symfony 8.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: OskarStark

demo/config/routes/framework.yaml

@@ -1,4 +1,4 @@
 when@dev:
     _errors:
-        resource: '@FrameworkBundle/Resources/config/routing/errors.xml'
+        resource: '@FrameworkBundle/Resources/config/routing/errors.php'
         prefix: /_error