add dependabot for github actions

.github/dependabot.yml

@@ -0,0 +1,8 @@
+# Dependabot configuration
+# ref: https://docs.github.com/en/code-security/supply-chain-security/keeping-your-dependencies-updated-automatically/configuration-options-for-dependency-updates
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"