Use EntityReferenceItemInterface::getReferenceableBundles() wherever core needs to discover reference-able entities for a field.

Problem/Motivation

EntityReferenceItemInterface::getReferenceableBundles() was introduced in #3057545: ResourceTypeRepository wrongly assumes that all entity reference fields have the setting "target_type" and deprecates entity reference fields that do not implement this interface; they must do so in Drupal 10.

This issue is a follow-up as foreseen in the original issue, to update any places in core where the field's settings are directly accessed, e.g. to determine the target_type. This will also improve compatibility with other field types, e.g. Dynamic Entity Reference.

ContentEntityBase::referencedEntities() will need updating, as it does:

> $property instanceof EntityReference

And generally, we should look through the whole codebase for anything which references either the EntityReference or EntityReferenceItem classes, or the entity_reference field type, and update them.

That could all be done in follow-ups though.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet