Disallow references to PRs that are closed but not merged

Library/Homebrew/formula_auditor.rb

@@ -830,8 +830,8 @@ module Homebrew
 
         issue = GitHub::API.open_rest("https://api.github.com/repos/#{owner}/#{repo}/issues/#{id}")
         next if issue.blank?
-        next if issue["pull_request"].present?
         next if issue["state"] == "open"
+        next if issue.dig("pull_request", "merged_at").present?
 
         issue_url = "https://github.com/#{owner}/#{repo}/#{type}/#{id}"
         problem "Formula refers to a GitHub issue or pull request that is closed: #{issue_url}"