chore(dependabot): remove explicit nuget registry

- Removes explicit NuGet registry configuration from Dependabot file.
- Relies on default registry settings to fetch NuGet updates, simplifying maintenance.

Author: winromulus

.github/dependabot.yaml

@@ -1,13 +1,7 @@
 version: 2
-registries:
-  public-nuget:
-    type: nuget-feed
-    url: https://api.nuget.org/v3/index.json
 updates:
   - package-ecosystem: nuget
     directory: "/"
-    registries:
-      - public-nuget
     schedule:
       interval: daily
     open-pull-requests-limit: 15