Bump phpunit/phpunit from 8.3.4 to 8.4.0

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 8.3.4 to 8.4.0.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-8.4.md)
- [Commits](sebastianbergmann/phpunit@8.3.4...8.4.0)

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