Switch Trello poster workflow to reusable workflow

Replaces the custom Trello poster workflow with a reusable workflow from alphagov/forms-deploy. This simplifies maintenance and ensures consistency with the main repository.

This also renames trello_poster.yml to trello-poster.yml for filename consistency.

Author: theseanything

.github/workflows/trello-poster.yml

@@ -0,0 +1,10 @@
+name: Trello Poster
+on:
+  pull_request:
+    types: [opened, edited]
+jobs:
+  trello-poster:
+    uses: alphagov/forms-deploy/.github/workflows/trello-poster.yml@main
+    secrets:
+      TRELLO_API_TOKEN: ${{ secrets.TRELLO_API_TOKEN }}
+      TRELLO_API_KEY: ${{ secrets.TRELLO_API_KEY }}