infra: add dependabot for github actions

.github/dependabot.yml

@@ -0,0 +1,12 @@
+# Set update schedule for GitHub Actions
+
+version: 2
+updates:
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      # Check for updates to GitHub Actions every week
+      interval: "monthly"
+    commit-message:
+      prefix: infra