Symfony 6 and PHPUnit 9.

[longwave]

Drupal uses stack/builder and so we need to add Symfony 6 compatibility here for us to be able to upgrade.

Locally the tests pass for me with these changes:

$ vendor/bin/phpunit
PHPUnit 9.5.11 by Sebastian Bergmann and contributors.

Warning:       Your XML configuration validates against a deprecated schema.
Suggestion:    Migrate your XML configuration using "--migrate-configuration"!

...............                                                   15 / 15 (100%)

Time: 00:00.021, Memory: 8.00 MB

OK (15 tests, 22 assertions)

However the new return type in StackedHttpKernel::handle() might mean this has to be a 2.0 release?

[fabpot]

It has to be a 2.0 release indeed.
But I would strongly recommend stopping using this and replacing it with standard Symfony classes instead.