build: Bump phpstan/phpstan

Bumps the dependencies group with 1 update in the /vendor-bin/phpstan directory: [phpstan/phpstan](https://github.com/phpstan/phpstan).


Updates `phpstan/phpstan` from 2.1.1 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@2.1.1...2.1.6)

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

Signed-off-by: dependabot[bot] <[email protected]>