github: Add dependabot to update GitHub Actions.

Add a dependabot.yml file so that workflows will be checked daily and Pull
Requests automatically raised when newer Actions are available.

.github/dependabot.yml

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