chore(composer): Allow PHPUnit 9 and Doctrine Instantiator 2

[scyzoryck]

Hello!

schmittjoh/serializer are requiring as a dev dependency this package. To run all tests with doctrine/instantiator in version 2, we need to also allow this dependency here. It looks like there is no backward incompatibilities between them - test passed on my local env with jackrabbit.
I also needed to bump PHPUnit to 9.x - in that version it produces some deprecation notices but it looks like the replacement method will be added in PHP 10.1. As example:

assertObjectHasAttribute() is deprecated and will be removed in PHPUnit 10. Refactor your test to use assertObjectHasProperty() (PHPUnit 10.1.0+) instead.

Best, scyzoryck.

[dbu]

thanks! lets see if the gitlab-ci agrees and if so, we can merge this.

note that phpcr-odm 2 is not released yet, i need to find time to wrap up the phpcr-odm bundle to check if there are any remaining issues. i hope to release sometime this month.