[Serializer] Fails to denormalize into php 8 mixed property

**Symfony version(s) affected**: 5.2.1

**How to reproduce**  
```php
final class DTO
{
    public mixed $a;
}

$serializer->deserialize('{"a": 1}', DTO::class, 'json');
```

gives error `The type of the "a" attribute for class "DTO" must be one of "" ("int" given).`

**Additional context**  
Works well if phpdoc `mixed` type is used instead.
