Bump phpstan/phpstan from 1.12.7 to 1.12.8 in /tools/phpstan

Bumps [phpstan/phpstan](https://github.com/phpstan/phpstan) from 1.12.7 to 1.12.8.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Changelog](https://github.com/phpstan/phpstan/blob/2.0.x/CHANGELOG.md)
- [Commits](phpstan/phpstan@1.12.7...1.12.8)

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

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