Make use of the new EntityInterface::getOriginal() method

Oops, there’s 📌 [PP-1] Compatibility with new `EntityInterface::[get|set]OriginalDefaultRevision()` Postponed already.

Nevermind, that other issue’s just about adding the methods to our own UnsavedIndex class to avoid fatal errors, not about using them in the rest of our code. So, re-opening.

This will now be mostly resolved in 🐛 Fix failing pipelines Active , but once we depend on Drupal 11.2 we can remove the DeprecationHelper::backwardsCompatibleCall() calls.