Force Dependabot ot use public NuGet feed

Otherwise it tries to use our CFS feed and can't check for updates.

Author: andyleejordan

.github/dependabot.yml

@@ -1,7 +1,13 @@
 version: 2
+registries:
+  nuget:
+    type: nuget-feed
+    url: https://api.nuget.org/v3/index.json
 updates:
 - package-ecosystem: nuget
   directory: "/"
+  registries:
+    - nuget
   labels: [ ]
   schedule:
     interval: weekly