DEV: Drop chat_uploads table and model and remove old references (#20926)

Followup to 0924f874bd9aca3515e17d0d084a35058e9d5757,
we migrated Chat::Upload records to UploadReference records
there and have not been making new Chat::Upload records
for some time, we can now delete the model and table.

plugins/chat/db/post_migrate/20230403012844_drop_chat_uploads.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class DropChatUploads < ActiveRecord::Migration[7.0]
+  DROPPED_TABLES ||= %i[chat_uploads]
+
+  def up
+    DROPPED_TABLES.each { |table| Migration::TableDropper.execute_drop(table) }
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end