Update phpstan/phpstan requirement from 1.10.15 to 1.10.21

Updates the requirements on [phpstan/phpstan](https://github.com/phpstan/phpstan) to permit the latest version.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Changelog](https://github.com/phpstan/phpstan/blob/1.11.x/CHANGELOG.md)
- [Commits](phpstan/phpstan@1.10.15...1.10.21)

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

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

composer.json

@@ -8,7 +8,7 @@
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "3.16.0",
-        "phpstan/phpstan": "1.10.15",
+        "phpstan/phpstan": "1.10.21",
         "phpunit/phpunit": "^9.5.10"
     },
     "autoload-dev": {