Update dependency phpunit/phpunit to v9

This MR contains the following updates:

Package	Type	Update	Change
phpunit/phpunit (source)	require-dev	major	8.5.5 -> 9.1.5

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Release Notes

sebastianbergmann/phpunit

v9.1.5

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v9.1.4

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v9.1.3

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v9.1.2

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v9.1.1

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v9.1.0

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v9.0.2

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v9.0.1

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v9.0.0

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