composer(deps-dev): bump phpunit/phpunit from 8.5.20 to 8.5.21

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 8.5.20 to 8.5.21.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-8.5.md)
- [Commits](sebastianbergmann/phpunit@8.5.20...8.5.21)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

composer.json

@@ -32,7 +32,7 @@
     "phpstan/phpstan-deprecation-rules": "~0.12.6",
     "phpstan/phpstan-phpunit": "~0.12.22",
     "phpstan/phpstan-strict-rules": "~0.12.11",
-    "phpunit/phpunit": "^8.5.20",
+    "phpunit/phpunit": "^8.5.21",
     "psalm/plugin-phpunit": "~0.16.1",
     "vimeo/psalm": "^4.10.0"
   },