Update phpunit/phpunit requirement from ^9.5.20 to ^9.5.21

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/main/ChangeLog-9.5.md)
- [Commits](sebastianbergmann/phpunit@9.5.20...9.5.21)

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

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

composer.json

@@ -30,7 +30,7 @@
         "mockery/mockery": "^1.5",
         "orchestra/testbench": "^6.28|^7.0",
         "orchestra/testbench-core": "^6.28|^7.0",
-        "phpunit/phpunit": "^9.5.20"
+        "phpunit/phpunit": "^9.5.21"
     },
     "autoload": {
         "psr-4": {