Bump phpunit/phpunit from 9.6.20 to 9.6.21 in /tools/phpunit

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 9.6.20 to 9.6.21.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/9.6.21/ChangeLog-9.6.md)
- [Commits](sebastianbergmann/phpunit@9.6.20...9.6.21)

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

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