ci: Add Dependabot to keep GitHub actions up to date

Dependabot will help you to keep you notified about new releases and help you update your workflows.

Interval is set to `monthly` so it won't make too much noise.

Author: jamacku

.github/dependabot.yml

@@ -0,0 +1,12 @@
+---
+
+version: 2
+
+updates:
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: monthly
+    labels:
+      - 'type: dependencies'
+      - 'github-actions'