Update dependency phpunit/phpunit to ^9.6.21

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
phpunit/phpunit (source)	^9.5 -> ^9.6.21

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Release Notes

sebastianbergmann/phpunit (phpunit/phpunit)

v9.6.21: PHPUnit 9.6.21

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Changed

• #​5956: Deprecation of the E_STRICT constant in PHP 8.4
• Removed .phpstorm.meta.php file as methods such as TestCase::createStub() use generics / template types for their return types and PhpStorm, for example, uses that information

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How to install or update PHPUnit

v9.6.20: PHPUnit 9.6.20

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Changed

• Updated dependencies (so that users that install using Composer's --prefer-lowest CLI option also get recent versions)

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How to install or update PHPUnit

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