Bring back Danger changelog iid check

The check to verify if the IID in changelog was set was
unintentionally removed in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/27588.
This reverts that change.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/212323

danger/changelog/Dangerfile

@@ -28,6 +28,8 @@ def check_changelog_yaml(path)
 
   if yaml["merge_request"].nil? && !helper.security_mr?
     message "Consider setting `merge_request` to #{gitlab.mr_json["iid"]} in #{gitlab.html_link(path)}. #{SEE_DOC}"
+  elsif yaml["merge_request"] != gitlab.mr_json["iid"]
+    fail "Merge request ID was not set to #{gitlab.mr_json["iid"]}! #{SEE_DOC}"
   end
 rescue Psych::SyntaxError, Psych::DisallowedClass, Psych::BadAlias
   # YAML could not be parsed, fail the build.