github-actions: enable dependabot (#17421)

Author: v1v

.github/dependabot.yml

@@ -0,0 +1,18 @@
+---
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directories:
+      - '/'
+      - '/.github/actions/*'
+    schedule:
+      interval: "weekly"
+      day: "sunday"
+      time: "22:00"
+    reviewers:
+      - "elastic/observablt-ci"
+      - "elastic/observablt-ci-contractors"
+    groups:
+      github-actions:
+        patterns:
+          - "*"