When we open a PR with conflicts, the PR owner gets a notification about the assignment but has no idea if this PR is with conflicts or not (in Scylla it's important since CI will not start on draft PR)
Let's add a comment to notify the user we have conflicts
Closes scylladb/scylladb#21939
(cherry picked from commit 2e6755ecca)
Closes scylladb/scylladb#22190