composer: update webmozart/assert requirement from ^1.10 to ^2.1

Updates the requirements on [webmozart/assert](https://github.com/webmozarts/assert) to permit the latest version.
- [Release notes](https://github.com/webmozarts/assert/releases)
- [Changelog](https://github.com/webmozarts/assert/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webmozarts/assert/commits)

---
updated-dependencies:
- dependency-name: webmozart/assert
  dependency-version: 2.1.1
  dependency-type: direct:development
...

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

Author: dependabot[bot]

composer.json

@@ -10,7 +10,7 @@
         "phpspec/phpspec": "^8.0",
         "sebastian/diff": "^4.0",
         "twig/twig": "^3.3",
-        "webmozart/assert": "^1.10"
+        "webmozart/assert": "^2.1"
     },
     "minimum-stability": "dev",
     "prefer-stable": true,