[XR] Update to 2018.3.0f1

Packages/manifest.json

 {
   "dependencies": {
-    "com.unity.ads": "2.0.8",
+    "com.unity.ads": "2.3.1",
-    "com.unity.collab-proxy": "1.2.11",
+    "com.unity.collab-proxy": "1.2.15",
-    "com.unity.purchasing": "2.0.1",
+    "com.unity.purchasing": "2.0.3",
     "com.unity.textmeshpro": "1.3.0",
     "com.unity.xr.arcore": "1.0.0-preview.23",
     "com.unity.xr.arfoundation": "1.0.0-preview.20",

ProjectSettings/ProjectVersion.txt

-m_EditorVersion: 2018.3.0b12
+m_EditorVersion: 2018.3.0f1