Update phpstan/phpstan-nette requirement from 0.12.17 to 0.12.21

Updates the requirements on [phpstan/phpstan-nette](https://github.com/phpstan/phpstan-nette) to permit the latest version.
- [Release notes](https://github.com/phpstan/phpstan-nette/releases)
- [Commits](phpstan/phpstan-nette@0.12.17...0.12.21)

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

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

composer.json

@@ -24,7 +24,7 @@
         "phpstan/phpstan": "0.12.90",
         "phpstan/phpstan-strict-rules": "0.12.9",
         "pepakriz/phpstan-exception-rules": "0.11.7",
-        "phpstan/phpstan-nette": "0.12.17",
+        "phpstan/phpstan-nette": "0.12.21",
         "nepada/phpstan-nette-tester": "0.3.1",
         "spaze/phpstan-disallowed-calls": "1.4.0",
         "php-parallel-lint/php-parallel-lint": "1.3.0",