Refresh Slack app home with notifications

What does this MR do and why?

This merge request updates the Slack app home content to reflect the design at #374139 (closed).

Screenshots or screen recordings

How to set up and validate locally

To QA this feature you must have access to the Ecosystem Test Slack Workspace.

1. Start a new GitPod instance and configure the GitLab for Slack (dev) app to work with it (docs)
2. Checkout this feature branch, bundle and yarn, and restart GDK (docs):

    cd /workspace/gitlab &&
        git fetch origin 374139-slack-app-home-update &&
        git checkout 374139-slack-app-home-update &&
        bundle &&
        yarn &&
        cd /workspace/gitlab-development-kit &&
        gdk restart

3. Visit /<PROJECT>/-/settings/integrations/gitlab_slack_application/edit and click Install GitLab for Slack app.
4. In the Ecosystem Test Slack Workspace, on the left-side Nav, click the App Home for the "GitLab (dev)" app (you may need to wait some seconds for your development environment to respond):

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #374139 (closed)