Update dependency phpunit/phpunit to v8.4.0

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
phpunit/phpunit (source)	require-dev	minor	8.3.5 -> 8.4.0

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Release Notes

sebastianbergmann/phpunit

### [`v8.4.0`](https://togithub.com/sebastianbergmann/phpunit/compare/8.3.5...8.4.0) [Compare Source](https://togithub.com/sebastianbergmann/phpunit/compare/8.3.5...8.4.0)

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