Update to 3.1.0-preview.8

Packages/manifest.json

     "com.unity.test-framework": "1.1.11",
     "com.unity.textmeshpro": "2.0.1",
     "com.unity.ugui": "1.0.0",
-    "com.unity.xr.arcore": "3.1.0-preview.7",
-    "com.unity.xr.arfoundation": "3.1.0-preview.7",
-    "com.unity.xr.arkit": "3.1.0-preview.7",
-    "com.unity.xr.arkit-face-tracking": "3.1.0-preview.7",
+    "com.unity.xr.arcore": "3.1.0-preview.8",
+    "com.unity.xr.arfoundation": "3.1.0-preview.8",
+    "com.unity.xr.arkit": "3.1.0-preview.8",
+    "com.unity.xr.arkit-face-tracking": "3.1.0-preview.8",
     "com.unity.xr.legacyinputhelpers": "2.0.8",
     "com.unity.modules.ai": "1.0.0",
     "com.unity.modules.androidjni": "1.0.0",