Add support for Symfony 8

[relthyg]

No description provided.

[romainnorberg]

+1 🙏

@relthyg If you're interested, I can help.

[relthyg]

Hey @romainnorberg,

I'm still on winter break and don't really have an overview of the situation at the moment. I'll probably get back to it next week. Feel free to get started if you feel like and have time.

[knallcharge]

I have tried to upgrade my project to Symfony 8 using this PR but found some issues:

1. There are some "Fatal error: Declaration of x must be compatible with y"-errors displayed.
2. Accessing the get()-method of an request was deprecated in Symfony 7.4 and will throw an "Attempted to call an undefined method named "get" of class "Symfony\Component\HttpFoundation\Request"."-error in Symfony 8.
3. XML-configs are deprecated since Symfony 7.4.

I have tried to fix the issues, you may want to compare my fork:
master...knallcharge:CraueFormFlowBundle:master

One thing I could not quite figure out: line 628 in Form/FormFlow.php where the instanceId is fetched from the request. Not being familiar with the internals of this bundle I didn't know if this is expected from the GET- or the POST-variables so I implemented a switch. Don't know if this is necessary, maybe some can advice?