Add dependabot (#2737)

.github/dependabot.yml

@@ -0,0 +1,13 @@
+version: 2
+updates:
+  - package-ecosystem: nuget
+    directory: "/"
+    schedule:
+      interval: daily
+    open-pull-requests-limit: 15
+    ignore:
+      # handled by Darc/Maestro flows
+      - dependency-name: "Microsoft.DotNet.Arcade.Sdk"
+      - dependency-name: Moq
+    commit-message:
+      prefix: '[main] '