`prs = response.json().get("items", [])` will return empty when there are no merged PRs, and this will just skip the all-label replacement process.
This is a regression following the work done in #19442
Adding another part to handle closed PRs (which is the majority of the cases we have in Scylla core)
Fixes: https://github.com/scylladb/scylladb/issues/19441
(cherry picked from commit efa94b06c2)
Closesscylladb/scylladb#19526
Today after Mergify opened a Backport PR, it will stay open until someone manually close the backport PR , also we can't track using labels which backport was done or not since there is no indication for that except digging into the PR and looking for a comment or a commit ref
The following changes were made in this PR:
* trigger add-label-when-promoted.yaml also when the push was made to `branch-x.y`
* Replace label `backport/x.y` with `backport/x.y-done` in the original PR (this will automatically update the original Issue as well)
* Add a comment on the backport PR and close it
Fixes: https://github.com/scylladb/scylladb/issues/19441
(cherry picked from commit 394cba3e4b)
Closesscylladb/scylladb#19495
