Add dependabot config

Author: christophfroehlich

.github/dependabot.yml

@@ -11,3 +11,10 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+  - package-ecosystem: "github-actions"
+    # Workflow files stored in the
+    # default location of `.github/workflows`
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    target-branch: "iron"