Change disabling method to avoid losing focus on facets

[JakePT]

Description of the Change

When facets are used in Instant Results the facet option checkboxes are disabled during loading to avoid a situation where the user selects two mutually incompatible options. This was being done using the disabled attribute, but the problem was that when this attribute was applied the checkboxes lost focus, meaning that keyboard users would lose their position.

This PR changes the checkbox to instead use aria-disabled for indicating the disabled status and to check the disabled status inside the onChange callback. This still prevents the user selecting mutually incompatible filters, but without losing keyboard focus on the checkbox,

Closes #3201

How to test the Change

1. Perform an Instant Results search.
2. Navigate to a facet checkbox with the keyboard and make a selection with Space.
3. After the new results are loaded attempt to navigate to the next or previous facet option. You should navigate to the expected element.
4. Attempt to quickly select two filter options. Once loading has started after selecting the first option it should not be possible to select another option until loading has finished.

Changelog Entry

Fixed - An issue where keyboard focus on a facet option was lost upon selection.

Credits

Props @JakePT

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.