Add pull_request_target event with unlabeled type to trigger-scylla-ci workflow. This allows automatic CI triggering when the 'conflicts' label is removed from a PR, in addition to the existing manual trigger via comment. The workflow now runs when: - A user posts a comment with '@scylladbbot trigger-ci' (existing) - The 'conflicts' label is removed from a PR (new) Fixes: https://scylladb.atlassian.net/browse/SCYLLADB-84 Closes scylladb/scylladb#27521