build(deps-dev): bump phpstan/phpstan from 2.1.18 to 2.1.21

Bumps [phpstan/phpstan](https://github.com/phpstan/phpstan) from 2.1.18 to 2.1.21.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Changelog](https://github.com/phpstan/phpstan/blob/2.1.x/CHANGELOG.md)
- [Commits](phpstan/phpstan@2.1.18...2.1.21)

---
updated-dependencies:
- dependency-name: phpstan/phpstan
  dependency-version: 2.1.21
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]