DEV: Make sure chat migration is in plugin directory (#29867)

This migration is for chat so it needs to live in the chat plugin
directory.

Follow up to: 23a7f00524b696847f8c127679c0413ac8f736f9

plugins/chat/db/migrate/20241120182858_add_index_to_chat_messages_on_chat_channel_id.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class AddIndexToChatMessagesOnChatChannelId < ActiveRecord::Migration[7.1]
+  disable_ddl_transaction!
+
+  def up
+    # Transaction has been disabled so we need to clean up the invalid index if index creation timeout
+    execute <<~SQL
+    DROP INDEX CONCURRENTLY IF EXISTS index_chat_messages_on_chat_channel_id_and_id
+    SQL
+
+    execute <<~SQL
+    CREATE INDEX CONCURRENTLY index_chat_messages_on_chat_channel_id_and_id
+    ON chat_messages (chat_channel_id,id)
+    WHERE deleted_at IS NOT NULL
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end