Symfony 8.0 removed support for XML configuration format entirely.
The XmlFileLoader class no longer exists in symfony/dependency-injection.

Changes:
- Convert services.xml to services.yaml
- Convert saga.xml to saga.yaml
- Convert serializers.xml to serializers.yaml
- Convert auditing.xml to auditing.yaml
- Update BroadwayExtension to use YamlFileLoader

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>