Add locale for deleted head branch (#26296)

As title.

It will be displayed in:

![image](https://github.com/go-gitea/gitea/assets/18380374/e8507a3b-14f4-4418-a347-a36689707a16)

options/locale/locale_en-US.ini

@@ -1784,6 +1784,8 @@ pulls.delete.text = Do you really want to delete this pull request? (This will p
 
 pulls.recently_pushed_new_branches = You pushed on branch <strong>%[1]s</strong> %[2]s
 
+pull.deleted_branch = (deleted):%s
+
 milestones.new = New Milestone
 milestones.closed = Closed %s
 milestones.update_ago = Updated %s