chore(modals): backdrop presentation role (#4934)

* chore(modals): aria-describedby and heading changes

* chore(modals): presentation role onto backdrop

* chore(modals): presentation role onto backdrop, actual

Author: kflournoy-sf

RELEASENOTES.md

@@ -97,6 +97,7 @@
 - Changed default header level from `h2` to `h1` for semantics and added more description around using headers in the Accessibility section.
 - Removed redundant `title` attribute from modal close button.
 - Updated use of `aria-describedby` attribute for modal content, and clarified in accessibility documentation section.
+- Added the `role="presentation"` attribute to the modal backdrop.
 
 ### [Pills](https://www.lightningdesignsystem.com/components/pills)
 ## Added