Webform calendar popup accessibility

[skoolbus39]

From @kmcguffey2 (unl/wdntemplates#1479):

The image for the calendar popup on Webforms is being flagged by Web Audit as an accessibility error.
“ARIA hidden element must not contain focusable elements”
Example: https://webaudit.unl.edu/sites/835/pages/10202664/

[macburgee1]

This behavior is by design: https://www.drupal.org/project/webform/issues/2534098.