Keep GitHub Actions up to date with GitHub's Dependabot

Like #78 but automated.

https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#package-ecosystem

.github/dependabot.yml

@@ -0,0 +1,10 @@
+version: 2
+updates:
+- package-ecosystem: "github-actions"
+  directory: "/"
+  groups:
+    GitHub_Actions:
+      patterns:
+        - "*"  # Group all Actions updates into a single larger pull request
+  schedule:
+    interval: weekly