[Slack Backlog app] Integration fix for Private Channel and Group DM

Because of the introduction of a new toolkit for Slack apps (article here), the Backlog team has updated the classic Slack app version of Backlog to the new Slack app.

With this update, you may face an integration issue in Slack where you are unable to receive Backlog updates in private channels or group DMs.

To resolve this issue, please refer to our Support Guide: Slack integration is not working for private channel or group direct message