ci: add dependabot for GH actions

Author: clason

.github/dependabot.yml

@@ -0,0 +1,16 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    cooldown:
+      default-days: 3
+    commit-message:
+      prefix: "ci"
+    labels:
+      - "CI"
+    groups:
+      actions:
+        patterns: ["*"]
+