Drupal 11 compatability

Ready for review.

Module was broken on 11.x after the NodeForm class has moved namespace: Drupal\node\NodeForm is now Drupal\node\Form\NodeForm →

Change record: https://www.drupal.org/node/3517871 →

The CR doesn't seem to imply we should need to do anything for BC as the issue references the use of the deprecation class loader. However - autoloading does not apply when we are type checking - i.e when we do instanceof it fails. And not fails like throws an error, it fails in that "$form_object instanceof NodeForm" evaluates to FALSE (as the class doesn't exist) and silently moves on.

Calling class_exists will trigger the autoloader and return to normal functionality (i.e allow this to work with 10.x and 11.x) but seeing as this is explicitly marked as an @internal class we shouldn't be using it at all. Given all the login was within an if statement that checked it was a node form, I've just changed to use the node form base form alter hook.