Add Dependabot config to monitor Actions versions

Check Actions versions weekly and create PRs to update them as necessary.

.github/dependabot.yaml

@@ -0,0 +1,13 @@
+# Dependabot config to enable checking for version updates on actions
+# and open pull requests to apply those updates
+# refs:
+#   https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot
+#   https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
+
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      # Check for updates to GitHub Actions every week
+      interval: "weekly"