feat(Timeline/TrendingLink): use open icon for open action

The previous icon made it hard to recognize what the action was supposed
to do. Additionally, the new one also indicate that it will take the
user to an external website.

mastodon/src/main/res/menu/trending_links_timeline.xml

@@ -2,7 +2,7 @@
 <menu xmlns:android="http://schemas.android.com/apk/res/android">
     <item
         android:id="@+id/open_link"
-        android:icon="@drawable/ic_fluent_document_one_page_24_filled"
+        android:icon="@drawable/ic_fluent_open_24_regular"
         android:showAsAction="always"
         android:title="@string/mo_trending_link_read"/>
 </menu>