Upgrade simplesamlphp/saml2 to ^4.20.3 and fix moved Chunk namespace

[monsefoster]

Summary

• Require PHP ^8.3 and simplesamlphp/saml2 ^4.20.3
• Update the moved XML Chunk namespace to SimpleSAML\XML\Chunk
• Align BridgeContainer method signatures with the updated SAML2 container contract
• Replace the unreachable phpcpd shim with systemsdk/phpcpd 8.0.0 for dev checks
• Update the integration test matrix to PHP 8.3/8.4 and clean stale PHPStan baseline entries

Verification

• composer update --no-interaction
• composer check-ci
• composer audit
• php -l src/SAML2/BridgeContainer.php
• ./ci/qa/phpstan