Discourage feature flags on security MRs

Modifies feature flags danger file to discourage the usafe of feature
flags on security merge requests.

As part of https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/1739

danger/feature_flag/Dangerfile

@@ -53,14 +53,22 @@ def message_for_feature_flag_with_group!(feature_flag:, mr_group_label:)
   end
 end
 
+def feature_flag_file_added?
+  feature_flag.feature_flag_files(change_type: :added).any?
+end
+
 def feature_flag_file_added_or_removed?
-  feature_flag.feature_flag_files(change_type: :added).any? || feature_flag.feature_flag_files(change_type: :deleted).any?
+  feature_flag_file_added? || feature_flag.feature_flag_files(change_type: :deleted).any?
 end
 
 feature_flag.feature_flag_files(change_type: :added).each do |feature_flag|
   check_feature_flag_yaml(feature_flag)
 end
 
+if helper.security_mr? && feature_flag_file_added?
+  fail "Feature flags are discouraged from security merge requests. Read the [security documentation](https://gitlab.com/gitlab-org/release/docs/-/blob/master/general/security/utilities/feature_flags.md) for details."
+end
+
 if feature_flag_file_added_or_removed?
   new_mr_title = helper.mr_title.dup
   new_mr_title << ' [RUN ALL RSPEC]' unless helper.run_all_rspec_mr?