[PHP 8.0] Add always class to AnnotationToAttribute to include string to ::class reference conversion

[TomasVotruba]

This way we can make sure the class references is used in places that expect it:

Before

#[\Doctrine\ORM\Mapping\Entity(repositoryClass: 'App\Some\SomeClass')]
class DoctrineEntity
{
}

After

#[\Doctrine\ORM\Mapping\Entity(repositoryClass: \App\Some\SomeClass::class)]
class DoctrineEntity
{
}