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-84Closesscylladb/scylladb#27521
(cherry picked from commit f7ffa395a8)
Closesscylladb/scylladb#27602