Refactor EntityReferenceItem to use HTMX instead of ['#ajax' => 'wrapper']

Problem/Motivation

core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php uses the Ajax API to dynamically update the field settings form when the handler is changed.

Proposed resolution

Refactor the form to use HTMX.

Remaining tasks

The existing functionality does not seem to have a test for the AJAX version of the functionality, specifically. A test will be needed.

The refactoring to HTMX, itself.

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet