Update serialization example with getSupportedTypes()

[Cocray]

Hello,

It'd be great to update the normalization example with the getSupportedTypes() method, as it is deprecated since Symfony 6.3 to not implement it.

I also think useful to explain how to do this part:

        // Make sure we're not called twice
        if (isset($context[self::ALREADY_CALLED])) {
            return false;
        }

when we have this:

    public function getSupportedTypes(?string $format): array
    {
        return [
            Book::class => true,
        ];
    }

Thanks :)

[Cocray]

Thank you @soyuka