add dependabot for gh actions

.github/dependabot.yml

@@ -0,0 +1,17 @@
+# Keeps the GitHub Actions referenced in .github/workflows up to date.
+# https://docs.github.com/code-security/dependabot/working-with-dependabot/dependabot-options-reference
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 5
+    labels:
+      - "dependencies"
+    # One PR per week for all action bumps instead of one PR per action.
+    groups:
+      github-actions:
+        patterns:
+          - "*"