Update simplesamlphp/xml-security requirement (#370)

Updates the requirements on [simplesamlphp/xml-security](https://github.com/simplesamlphp/xml-security) to permit the latest version.

Updates `simplesamlphp/xml-security` to 1.12.0
- [Release notes](https://github.com/simplesamlphp/xml-security/releases)
- [Commits](simplesamlphp/xml-security@v1.11.0...v1.12.0)

---
updated-dependencies:
- dependency-name: simplesamlphp/xml-security
  dependency-type: direct:production
  dependency-group: production-dependencies
...

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composer.json

@@ -33,7 +33,7 @@
         "psr/log": "^2.0 || ^3.0",
         "simplesamlphp/assert": "~1.6.0",
         "simplesamlphp/xml-common": "~1.22.0",
-        "simplesamlphp/xml-security": "~1.11.0",
+        "simplesamlphp/xml-security": "~1.12.0",
         "simplesamlphp/xml-soap": "~1.5.6"
     },
     "require-dev": {