fix: use cascade 'merge' only if available

[fbuchlak]

Closes #973.

@VincentLanglet Check is not performed on EntityManagerInterface because there is no merge method declared in supported doctrine/orm versions. Previously it was inherited from ObjectManager in doctrine/persistence, but it was removed in persistence 3.0.

Changelog

### Fixed
- Use cascade `merge` only if it's available in `doctrine/orm`

[VincentLanglet]

You will lose the merge for ODM user.
https://github.com/doctrine/mongodb-odm/blob/2.10.x/lib/Doctrine/ODM/MongoDB/DocumentManager.php

it should be

if (
    class_exists(EntityManager::class) && method_exists(EntityManager::class, 'merge')
    || class_exists(DocumentManager::class) && method_exists(DocumentManager::class, 'merge')
)

but I assume it will create issue when both lib are installed.

So we have to know which entity we extends.

Maybe

$classParents = class_parents($config['class']['category']);

if (\in_array(Entity/BaseCategory::class, $classParents, true) && class_exists(EntityManager::class) && !method_exists(EntityManager::class, 'merge')) {
     unset($categoryCascade['merge']);
}

?

[fbuchlak]

Sorry for late reply. I've checked odm src and it seems like merge isn't deprecated (even in 3.0 branch), so 2nd option should be just fine.

[VincentLanglet]

Thanks, released in https://github.com/sonata-project/SonataClassificationBundle/releases/tag/4.9.1