chore: bump phpunit/phpunit from 9.6.19 to 9.6.20 in the all-deps group

Bumps the all-deps group with 1 update: [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit).


Updates `phpunit/phpunit` from 9.6.19 to 9.6.20
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/9.6.20/ChangeLog-9.6.md)
- [Commits](sebastianbergmann/phpunit@9.6.19...9.6.20)

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

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