Support Symfony 8

Symfony 8 will be out later this month.

I can submit a PR that updates composer.json and the CI and see how far we get, unless someone with more experience has a plan to do so. Usually I get caught up with the test configuration for the different versions.

Work is in progress in #3090, but I need to solve #3088 first to make this usable for ORM users (that's the reason I haven't merged this PR to be included in my 4.1.0 release, delaying it to a 4.2.0 release once it is solved).