[MA-34]: Add state information (#29695)

Co-authored-by: Mattermost Build <build@mattermost.com>

webapp/channels/src/components/sidebar/sidebar_category_header.tsx

@@ -60,6 +60,7 @@ export const SidebarCategoryHeader = React.forwardRef((props: Props, ref?: React
                 className={classNames('SidebarChannelGroupHeader_groupButton')}
                 aria-label={props.displayName}
                 onClick={props.onClick}
+                aria-expanded={!props.isCollapsed}
             >
                 <i
                     className={classNames('icon icon-chevron-down', {