Update phpstan/phpstan requirement from ^1.12 to ^2.0

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/2.0.x/CHANGELOG.md)
- [Commits](phpstan/phpstan@1.12.0...2.0.2)

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

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

composer.json

@@ -20,7 +20,7 @@
     "phpbench/phpbench": "^1.3",
     "saggre/phpdocumentor-markdown": "^0.1",
     "tomasvotruba/lines": "^0.6.0",
-    "phpstan/phpstan": "^1.12"
+    "phpstan/phpstan": "^2.0"
   },
   "autoload": {
     "psr-4": { "MammothPHP\\WoollyM\\": "src/"}