chore: add unlabeled trigger to changelog-preview (#5315)

This PR adds the `unlabeled` trigger to the changelog preview workflow
to ensure the preview is updated when a label is removed.

Author: BYK

.github/workflows/changelog-preview.yml

@@ -7,6 +7,7 @@ on:
     - reopened
     - edited
     - labeled
+    - unlabeled
 permissions:
   contents: write
   pull-requests: write