chore(ci): set the milestone when a pull request is closed (take 4)

The milestone can only be determined to be final when a pull request
is merged.

It is possible that a pull request is opened during the development of
v10 and merged after it is published.

It is also possible that it is permanently closed without being merged.

(cherry picked from commit 6f53f7d007)

.forgejo/workflows/milestone.yml

@@ -6,11 +6,11 @@ name: milestone
 on:
   pull_request_target:
     types:
-      - opened
+      - closed
 
 jobs:
   set:
-    if: vars.ROLE == 'forgejo-coding'
+    if: vars.ROLE == 'forgejo-coding' && github.event.pull_request.merged
     runs-on: docker
     container:
       image: 'code.forgejo.org/oci/ci:1'