Update ARKit face tracking to preview 6.

preview.5 never made it to production due to an invalid dependency.

Packages/manifest.json

     "com.unity.xr.arcore": "2.1.0-preview.5",
     "com.unity.xr.arfoundation": "2.1.0-preview.3",
     "com.unity.xr.arkit": "2.1.0-preview.5",
-    "com.unity.xr.arkit-face-tracking": "1.0.0-preview.5",
+    "com.unity.xr.arkit-face-tracking": "1.0.0-preview.6",
     "com.unity.modules.ai": "1.0.0",
     "com.unity.modules.animation": "1.0.0",
     "com.unity.modules.assetbundle": "1.0.0",