ci: fix some minor issues in the backport ci job

-Due to a newer version we need to use add_labels instead of just
 labels

-The backport PR will now also copy the milestones in case the
 milstones were set

.github/workflows/backport.yml

@@ -112,7 +112,10 @@ jobs:
 
           # Automatically add labels to backport PRs.
           # The 'no-changelog' label skips the release notes check in CI.
-          labels: no-changelog
+          add_labels: no-changelog
+
+          # Copy milestone from original PR to backport PR.
+          copy_milestone: true
 
           # Merge strategy - skip merge commits, use cherry-pick only.
           merge_commits: skip