Bump phpstan/phpstan from 2.1.17 to 2.1.21 in the patch-updates group (#54)

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


Updates `phpstan/phpstan` from 2.1.17 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@2.1.17...2.1.21)

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

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Author: dependabot[bot]