Update the Element Call unstable URL

We've switched from Netlify to an in-house Kubernetes deployment with a new URL.

element.io/nightly/config.json

@@ -53,7 +53,7 @@
         "feature_video_rooms": true
     },
     "element_call": {
-        "url": "https://element-call-livekit.netlify.app"
+        "url": "https://call.element.dev"
     },
     "map_style_url": "https://api.maptiler.com/maps/streets/style.json?key=fU3vlMsMn4Jb6dnEIFsx"
 }