Add aria-expanded to content warning toggle button (#19975)

Fixes #19920

app/javascript/mastodon/components/status_content.js

@@ -258,7 +258,7 @@ class StatusContent extends React.PureComponent {
           <p style={{ marginBottom: hidden && status.get('mentions').isEmpty() ? '0px' : null }}>
             <span dangerouslySetInnerHTML={spoilerContent} className='translate' lang={lang} />
             {' '}
-            <button tabIndex='0' className={`status__content__spoiler-link ${hidden ? 'status__content__spoiler-link--show-more' : 'status__content__spoiler-link--show-less'}`} onClick={this.handleSpoilerClick}>{toggleText}</button>
+            <button type='button' className={`status__content__spoiler-link ${hidden ? 'status__content__spoiler-link--show-more' : 'status__content__spoiler-link--show-less'}`} onClick={this.handleSpoilerClick} aria-expanded={!hidden}>{toggleText}</button>
           </p>
 
           {mentionsPlaceholder}