Bump phpstan/phpstan in the development-dependencies group (#29)

Bumps the development-dependencies group with 1 update: [phpstan/phpstan](https://github.com/phpstan/phpstan).


Updates `phpstan/phpstan` from 2.1.2 to 2.1.6
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Changelog](https://github.com/phpstan/phpstan/blob/2.1.x/CHANGELOG.md)
- [Commits](phpstan/phpstan@2.1.2...2.1.6)

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

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

Author: dependabot[bot]