build(deps-dev): Bump phpunit/phpunit from 9.6.13 to 9.6.15

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 9.6.13 to 9.6.15.
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/9.6.15/ChangeLog-9.6.md)
- [Commits](sebastianbergmann/phpunit@9.6.13...9.6.15)

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

Signed-off-by: dependabot[bot] <support@github.com>