Update simplesamlphp/assert requirement (#368)

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

Updates `simplesamlphp/assert` to 1.6.0
- [Release notes](https://github.com/simplesamlphp/assert/releases)
- [Commits](simplesamlphp/assert@v1.5.0...v1.6.0)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

composer.json

@@ -31,7 +31,7 @@
         "psr/clock": "^1.0",
         "psr/http-message": "^2.0",
         "psr/log": "^2.0 || ^3.0",
-        "simplesamlphp/assert": "~1.5.0",
+        "simplesamlphp/assert": "~1.6.0",
         "simplesamlphp/xml-common": "~1.20.0",
         "simplesamlphp/xml-security": "~1.10.0",
         "simplesamlphp/xml-soap": "~1.5.5"