Drupal 10 and Symfony 6 support

Alright, so I think I've fixed the blockers to this working and have it running again.

There's still a difficult to pin down bug that sometimes causes an exception in the JavaScript on popping state by hitting back when it tries to construct a State object but passes null as the historyPosition parameter due to an incorrect sessionStorage ID. Given that it only sometimes happens and we're planning on replacing the homegrown JavaScript with Hotwire Turbo which is expected to handle this for us, I'm not going to try and fix it but rather focus on the port instead.

Automatically closed - issue fixed for 2 weeks with no activity.