Bump phpstan/phpstan from 0.12.19 to 0.12.20 in /dev-tools (#36)

Bumps [phpstan/phpstan](https://github.com/phpstan/phpstan) from 0.12.19 to 0.12.20.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Commits](phpstan/phpstan@0.12.19...0.12.20)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

dev-tools/composer.json

@@ -6,7 +6,7 @@
         "maglnet/composer-require-checker": "^2.1.0",
         "mi-schi/phpmd-extension": "^4.3.0",
         "phpmd/phpmd": "^2.8.2",
-        "phpstan/phpstan": "^0.12.19",
+        "phpstan/phpstan": "^0.12.20",
         "phpstan/phpstan-strict-rules": "^0.12.2",
         "squizlabs/php_codesniffer": "^3.5.5",
         "vimeo/psalm": "^3.11.2",