Automate the process for self-managed to update their GitLab for Slack app manifest

This issue is part of the epic &1211 (closed).

Problem

In #358872 (closed) we delivered an MVC for self-managed to install, and update, their own copies of the GitLab for Slack app.

The MVC relies on some manual steps, particularly when it comes to updating their Slack app with the new manifest.

Ideally, we could automate as much of this process as possible.

Proposal

There is no concrete proposal yet. We should look into Slack App Manifest APIs and determine how we could update their Slack app's manifest in the most automated flow possible.

Implementation proposal by Bojan: #401920 (comment 1755095460)