Update PHPUnit to a newer version

[yorkshire-pudding]

The bee tests currently use PHPUnit 8
According to https://phpunit.de/supported-versions.html PHPUnit 8 reached the end of bugfix support in February 2023 though it is not yet end of life and I think might still be getting security releases.

Major Version	PHP Compatibility	Initial Release	End of Bugfix Support	End of Life
PHPUnit 10	>= PHP 8.1	February 3, 2023	February 7, 2025	To be determined
PHPUnit 9	>= PHP 7.3	February 7, 2020	February 2, 2024	To be determined
PHPUnit 8	>= PHP 7.2	February 1, 2019	February 3, 2023	To be determined

Table copied on 2023-09-21

I think it would be reasonable to move to PHPUnit 9 but I don't think we're ready for 10 as there are probably quite a few sites still with PHP 7.4 that the bee testing couldn't cover.

Need to test whether using PHPUnit 9 causes any issues. There are some Backward Incompatibilities that might need addressing: https://phpunit.de/announcements/phpunit-9.html

[yorkshire-pudding]

assertRegExp() was deprecated in 2020 for version 9.1 - sebastianbergmann/phpunit#4086

Replace with assertMatchesRegularExpression()