Mark correctly as read reverse-order announcements (#15070)

This fixes a bug in #15065 where the "read" indicator was not getting correctly set. The ID of a dismissed announcement is now correct.

app/javascript/mastodon/features/getting_started/components/announcements.js

@@ -396,7 +396,7 @@ class Announcements extends ImmutablePureComponent {
   _markAnnouncementAsRead () {
     const { dismissAnnouncement, announcements } = this.props;
     const { index } = this.state;
-    const announcement = announcements.get(index);
+    const announcement = announcements.get(announcements.size - 1 - index);
     if (!announcement.get('read')) dismissAnnouncement(announcement.get('id'));
   }