Update phpstan/phpstan requirement from 1.12.5 to 1.12.6 (#733)

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@1.12.5...1.12.6)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Diff for: composer.json

@@ -29,7 +29,7 @@
     "require-dev": {
         "php-parallel-lint/php-parallel-lint": "1.4.0",
         "phpstan/extension-installer": "1.4.3",
-        "phpstan/phpstan": "1.12.5",
+        "phpstan/phpstan": "1.12.6",
         "phpstan/phpstan-phpunit": "1.4.0",
         "phpunit/phpunit": "8.5.38",
         "rector/rector": "1.2.6"