Merge pull request #570 from Cysharp/ci/dependabot

ci: add depndabot.yaml to update github-actions

.github/dependabot.yaml

@@ -0,0 +1,7 @@
+# ref: https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly" # Check for updates to GitHub Actions every week