When a backport PR is promoted to the release branch, we automatically close the backport PR (since GitHub will only close the one based on the default branch) and update the labels in the original PRs
In a situation when we have multiple `closes` prefixes, the script will use the first one (which is not the correct one), see 3ddb61c90e
Fixing this by always using the last line with the `closes` prefix
Closes scylladb/scylladb#21498