Preserve PR labels when backporting

This should make release notes containing backports to be
better sorted.

Author: berland

.github/workflows/backport.yml

@@ -33,4 +33,5 @@ jobs:
         uses: korthout/backport-action@7c3f6cd5843cac11bc59a04a1b7699af93261670 # v4.5.0
         with:
           add_author_as_assignee: true
+          copy_labels_pattern: release-notes:.*
           github_token: ${{ steps.generate-token.outputs.token }}