Add support for symfony/process 6

[aryehraber]

This PR bumps the composer dependencies to support the latest symfony/process package (which is a downstream dependency of Laravel 9). Since symfony/process requires PHP 8, I also had to bump PHPUnit and update some deprecated tests.

The following tests returned as "risky" and I'm not entirely sure how to fix them:

There were 3 risky tests:

1) Nesk\Rialto\Tests\ImplementationTest::forbidden_options_are_removed
This test did not perform any assertions

/Users/aryeh/Code/Packages/rialto/tests/ImplementationTest.php:369

2) Nesk\Rialto\Tests\ImplementationTest::logger_is_used_when_provided
This test did not perform any assertions

/Users/aryeh/Code/Packages/rialto/tests/ImplementationTest.php:464

3) Nesk\Rialto\Tests\ImplementationTest::node_console_calls_are_logged
This test did not perform any assertions

Thank you greatly for this great package (we use PuPHPeteer extensively), and please let me know if there are any questions or feedback!

[Luukdewaaier]

@nesk any chance you can give this a look? Would love to upgrade my projects

[seifti]

@nesk @rerzy @spekulatius @peter279k @kip-13 any chance that this can be committed?

[peter279k]

It looks good, but the CI is not running on this PR. I'm not sure this changes can be worked successfully.