.asf.yaml: Delete branches when pull requests are merged

https://github.com/apache/buildstream/branches/all shows a lot of
branches that have already been merged into master. They should perhaps
be cleaned up separately, but enable the `del_branch_on_merge` flag to
automatically delete such branches in the future.

Author: cs-shadow

.asf.yaml

@@ -33,6 +33,9 @@ github:
     # disable rebase button:
     rebase: false
 
+  # Close branches when pull requests are merged
+  del_branch_on_merge: true
+
   # Enable pages publishing
   ghp_branch: gh-pages
   ghp_path: /