Add dotnet-sdk package ecosystem to dependabot configuration (#830)

Author: Copilot

.github/dependabot.yml

@@ -33,3 +33,10 @@ updates:
     # Check for updates once a week
     schedule:
       interval: "weekly"
+
+  - package-ecosystem: "dotnet-sdk"
+    # Look for a `global.json` in the `root` directory
+    directory: "/"
+    # Check for updates once a week
+    schedule:
+      interval: "weekly"