Reconsider disabling ajax element triggers in beforeSend

Problem/Motivation

Whenever an element is an AJAX trigger, Drupal.Ajax.prototype.beforeSend applies the disabled HTML attribute to it.

This may prove to be troublesome for some keyboard users in certain form configurations, as these elements become non-interactive.

Steps to reproduce

1. Set up a form with a text field that triggers a long-running AJAX request on blur and a submit button
2. Tab to the text field
3. Fill out the text field
4. Tab to the submit button
5. Press shift+tab before the AJAX request has completed. Notice that focus is now outside of the form entirely, as disabled elements cannot receive focus.

Proposed resolution

Might it be more appropriate to apply aria-disabled here instead and to prevent value changes in another way?

Remaining tasks

• See if this is even something to consider (calling all a11y maintainers!)
• See if this is even feasible due to b/c concerns.
• Do it(?)

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

TBD