Bump phpstan/phpstan from 1.12.16 to 2.1.5 in /tools

Bumps [phpstan/phpstan](https://github.com/phpstan/phpstan) from 1.12.16 to 2.1.5.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Changelog](https://github.com/phpstan/phpstan/blob/2.1.x/CHANGELOG.md)
- [Commits](phpstan/phpstan@1.12.16...2.1.5)

---
updated-dependencies:
- dependency-name: phpstan/phpstan
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

tools/composer.json

@@ -3,7 +3,7 @@
     "description": "Aeon PHP Calendar - Tools",
     "require-dev": {
         "phpunit/phpunit": "^9.5",
-        "phpstan/phpstan": "^1.1.2",
+        "phpstan/phpstan": "^2.1.5",
         "infection/infection": "^0.29.10",
         "phpbench/phpbench": "^1.2.6",
         "friendsofphp/php-cs-fixer": "^3.1",