cicd: use renovate instead of dependabot.

Author: Codex-

.github/dependabot.yml

@@ -0,0 +1,11 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": ["config:base", ":preserveSemverRanges"],
+  "timezone": "Pacific/Auckland",
+  "labels": ["dependencies"],
+  "vulnerabilityAlerts": {
+    "labels": ["security"],
+    "automerge": true,
+    "assignees": ["@codex-"]
+  }
+}