build(deps-dev): bump phpstan/phpstan-phpunit from 1.3.14 to 1.3.15

Bumps [phpstan/phpstan-phpunit](https://github.com/phpstan/phpstan-phpunit) from 1.3.14 to 1.3.15.
- [Release notes](https://github.com/phpstan/phpstan-phpunit/releases)
- [Commits](phpstan/phpstan-phpunit@1.3.14...1.3.15)

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

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