meta: add `dependabot.yml` to keep GHA up-to-date (#5083)

.github/dependabot.yml

@@ -0,0 +1,9 @@
+# Basic set up for three package managers
+
+version: 2
+updates:
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: 'github-actions'
+    directory: '/'
+    schedule:
+      interval: 'weekly'