[release]: Update Flatpak manifest

Author: mr-cheffy

app.zen_browser.zen.metainfo.xml

@@ -59,7 +59,7 @@
     <control>keyboard</control>
   </supports>
   <releases>
-    <release version="1.21.16b" date="2026-08-28T09:01:48.059Z">
+    <release version="1.21.16b" date="2026-08-29T21:30:24.830Z">
       <url type="details">https://zen-browser.app/release-notes/1.21.16b</url>
     </release>
     <release version="1.21.15b" date="2026-08-19T17:14:19.848Z">