Submit GitLab Slack app for Slack app review (but not publish it yet)

Submit the GitLab Slack app for review by the Slack Review team, following this guide https://gitlab.com/gitlab-org/manage/integrations/team/-/blob/main/integrations/slack.md#publish-changes-to-the-gitlab-slack-app.

We will not publish the app yet once it is approved, instead, we will publish it at the same time that we enable the flag #381012 (closed).