refactor(CAccordion): add aria-expanded

Author: mrholek

docs/4.0/components/CAccordion.mdx

@@ -150,8 +150,6 @@ Add `flush` to remove the default `background-color`, some borders, and some rou
   }}
 </Playground>
 
-<!-- ## TODO: Accessibility - https://coreui.io/docs/components/accordion/#accessibility -->
-
 ## API
 
 ### CAccordion

src/components/accordion/CAccordionButton.tsx

@@ -18,7 +18,7 @@ export const CAccordionButton = forwardRef<HTMLButtonElement, CAccordionButtonPr
     const _className = classNames('accordion-button', { collapsed: collapsed }, className)
 
     return (
-      <button className={_className} {...rest} ref={ref}>
+      <button className={_className} {...rest} aria-expanded={!collapsed} ref={ref}>
         {children}
       </button>
     )