appimage: bump Qt to 6.8.3 (trying to fix audio stutters)

Author: jakubfi

.github/workflows/appimage.yml

@@ -40,11 +40,13 @@ jobs:
 
       # jammy's Qt is 6.2, whose QSoundEffect rebuilds its GStreamer pipeline per
       # play and clips the attack. A newer Qt uses the self-contained FFmpeg
-      # backend (no GStreamer element bundling) and a persistent audio sink.
+      # backend (no GStreamer element bundling). Not older than 6.8: up to 6.7
+      # QSoundEffect reopens its pulse stream per play and the prebuffer race
+      # makes panel sounds stutter and cuts out the PSU noise loop.
       - name: Install Qt
         uses: jurplel/install-qt-action@v4
         with:
-          version: '6.7.3'
+          version: '6.8.3'
           modules: 'qtmultimedia'
           cache: true