[Symfony 6.4] Add DataTransformerInterface return types

[TomasVotruba]

Ref #537

[trsteel88]

@TomasVotruba is this forcing a return type on DataTransformers in symfony?

I have the following

public function reverseTransform(mixed $value): ?string
    {
        if (null === $value || '' === (string) $value) {
            return null;
        }

        return 'foo';
    }

But rector is changing this to:

public function reverseTransform(mixed $value): object|string
    {
        if (null === $value || '' === (string) $value) {
            return null;
        }

        return 'foo';
    }

But this isn't correct. It can only be ?string so now I have PHPStan failing on me:

::reverseTransform() should return object|string but returns null.  

[trsteel88]

Ah using the 71 set instead of 64 fixed it.

$rectorConfig->import(Rector\Symfony\Set\SymfonySetList::SYMFONY_71);