Update simplesamlphp/assert requirement

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

Updates `simplesamlphp/assert` to 1.8.1
- [Release notes](https://github.com/simplesamlphp/assert/releases)
- [Commits](simplesamlphp/assert@v1.7.0...v1.8.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

composer.json

@@ -17,7 +17,7 @@
 
         "composer/composer": "~2.8.3",
         "composer-plugin-api": "~2.6.0",
-        "simplesamlphp/assert": "~1.7.0"
+        "simplesamlphp/assert": "~1.8.1"
     },
     "require-dev": {
         "simplesamlphp/simplesamlphp-test-framework": "~1.8.0"