Update simplesamlphp/xml-common requirement

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

Updates `simplesamlphp/xml-common` to 1.26.0
- [Release notes](https://github.com/simplesamlphp/xml-common/releases)
- [Commits](simplesamlphp/xml-common@v1.25.0...v1.26.0)

---
updated-dependencies:
- dependency-name: simplesamlphp/xml-common
  dependency-version: 1.26.0
  dependency-type: direct:production
  dependency-group: production-dependencies
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

composer.json

@@ -18,7 +18,7 @@
 
         "simplesamlphp/assert": "~1.8.1",
         "simplesamlphp/saml2": "~5.0.0",
-        "simplesamlphp/xml-common": "~1.25.0"
+        "simplesamlphp/xml-common": "~1.26.0"
     },
     "require-dev": {
         "mockery/mockery": "~1.6.12",