DEV: Update importers from PostUpload to UploadReference (#23681)

Discourse stopped using PostUpload in https://github.com/discourse/discourse/commit/9db8f00b3dd6f2881adf1b786e29426889225e7a. Since then, these importers have been writing to the table, but any data was totally unused. This commit updates the easy cases to use UploadReference, and adds an error to the discourse_merger import script, which needs more significant work.

script/import_scripts/higher_logic.rb

@@ -189,9 +189,7 @@ class ImportScripts::HigherLogic < ImportScripts::Base
 
               post.raw << "\n\n" << html
               post.save!
-              unless PostUpload.where(post: post, upload: upload).exists?
-                PostUpload.create!(post: post, upload: upload)
-              end
+              UploadReference.ensure_exist!(upload_ids: [upload.id], target: post)
             end
           end
         end