Update phpunit/phpunit requirement from ^9.6.15 to ^10.5.21

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/10.5.21/ChangeLog-10.5.md)
- [Commits](sebastianbergmann/phpunit@9.6.15...10.5.21)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-type: direct:development
...

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

composer.json

@@ -16,7 +16,7 @@
         "psr/http-server-middleware": "^1.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^9.6.15",
+        "phpunit/phpunit": "^10.5.21",
         "laminas/laminas-diactoros": "^3.3.0",
         "phpstan/phpstan": "^1.10.55",
         "phpstan/phpstan-phpunit": "^1.3.15",