Enable GitLab for Slack app for Instances and Groups

Release notes

Problem to solve

In order to fully deprecate the Slack Notifications integration and the Slack Slash Commands integration #358875 to complete the effort to combine their features into the GitLab for Slack app, we will need to provide support for Instances and Groups to enable and manage the app.

Currently, the GitLab for Slack app is only a project-level integration:

Integration	Instance	Group	Project
Slack Notifications Integration (deprecated)	✅	✅	✅
Slack Slash Commands Integration	✅	✅	✅
GitLab for Slack app			✅

Proposal

Documentation

When this feature has been implemented we can revert the docs change in !134214 (merged) as group mention notifications will be supported.

Intended users

Feature Usage Metrics

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