Composer(deps-dev): Bump php-stubs/wordpress-stubs from 5.9.6 to 5.9.9

Bumps [php-stubs/wordpress-stubs](https://github.com/php-stubs/wordpress-stubs) from 5.9.6 to 5.9.9.
- [Release notes](https://github.com/php-stubs/wordpress-stubs/releases)
- [Commits](php-stubs/wordpress-stubs@v5.9.6...v5.9.9)

---
updated-dependencies:
- dependency-name: php-stubs/wordpress-stubs
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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