Update friendsofphp/php-cs-fixer requirement from 3.20.0 to 3.21.0 (#74)

Updates the requirements on [friendsofphp/php-cs-fixer](https://github.com/PHP-CS-Fixer/PHP-CS-Fixer) to permit the latest version.
- [Release notes](https://github.com/PHP-CS-Fixer/PHP-CS-Fixer/releases)
- [Changelog](https://github.com/PHP-CS-Fixer/PHP-CS-Fixer/blob/master/CHANGELOG.md)
- [Commits](PHP-CS-Fixer/PHP-CS-Fixer@v3.20.0...v3.21.0)

---
updated-dependencies:
- dependency-name: friendsofphp/php-cs-fixer
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

composer.json

@@ -8,7 +8,7 @@
         "phpat/phpat": "^0.10",
         "facile-it/facile-coding-standard": "0.5.2",
         "vimeo/psalm": "4.30.0",
-        "friendsofphp/php-cs-fixer": "3.20.0",
+        "friendsofphp/php-cs-fixer": "3.21.0",
         "phpstan/phpstan": "^1.8",
         "rector/rector": "^0.16.0 || ^0.17.0"
     },