Regression in #2940339: when multiple vocabularies exist, normalization of Terms fails

Problem/Motivation

Discovered via #2977848: Update to JSON API 1.20.

#2940339: Port reference field support for non-empty entity reference fields not pointing to an entity from #2543726 added this:

    if ($target_entity === NULL) {
      $host_entity = $parent->getHostEntity();
      $relatable_resource_types = $resource_type_repository->get(
        $host_entity->getEntityTypeId(),
        $host_entity->bundle()
      )->getRelatableResourceTypes()[$parent->getPropertyName()];
      if (count($relatable_resource_types) !== 1) {
        throw new \RuntimeException('Relationships to virtual resources are possible only if a single resource type is relatable.');
      }

Having multiple vocabularies causes $relatable_resource_types['parent'] to have multiple resource types. But why?

Turns out that the parent field definition does NOT restrict the parent to be in the same bundle. That means that the Politics term from the Tags vocabulary could have BMW from the Cars vocabulary as its parent term. This of course makes zero sense.

Proposed resolution

1. Work around this in the JSON API module.
2. Fix it in Drupal core.

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

Term entity type's parent field definition is now more precise.