chore: enable Dependabot for security updates only (#279)

.github/dependabot.yml

@@ -0,0 +1,20 @@
+# NOTE: This file is not managed by Projen because if you enable Dependabot through Projen,
+# it will delete the upgrade-main job and expect you to only use Dependabot for updates.
+# That is not what we want either; we just want to use Dependabot for security updates.
+
+version: 2
+updates:
+  - package-ecosystem: npm
+    versioning-strategy: lockfile-only
+    directory: /
+    schedule:
+      interval: daily
+    ignore:
+      - dependency-name: projen
+    labels:
+      - auto-approve
+      - automerge
+      - dependencies
+      - security
+    # Disable version updates for npm dependencies, only use Dependabot for security updates
+    open-pull-requests-limit: 0