ci: Enable Dependabot version updates for GitHub Actions (#1900)

* Add GitHub Dependabot config to check for GitHub Action version updates and
to open PRs if available. Set the check and update interval to weekly.
   - c.f. https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

.github/dependabot.yml

@@ -0,0 +1,7 @@
+version: 2
+updates:
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"