Deprecate Slack notifications integration
## Context This came out of a [conversation](https://gitlab.com/gitlab-org/gitlab/-/issues/373321#note_1116904617) on https://gitlab.com/gitlab-org/gitlab/-/issues/373321 ## Background There are currently 2 Slack integrations: * Slack notifications: set up starting point is in Slack app. Sends notifications from GitLab to Slack * Slack application: set up starting point is GitLab. Allows users to type `/commands` to do actions in GitLab Slack notifications will be deprecated and its functionality will be brought into Slack application. We will soon be deprecating Slack notifications (full context as to why in [epic](https://gitlab.com/groups/gitlab-org/-/epics/8670#why-we-need-to-deprecate-slack-notifications)). Eventually, users of Slack notifications will be able to migrate from Slack notifications to Slack application. Their notifications will be brought over. As an intermediary step, we will be adding notifications in the Slack application integration. ## Problem * When notifications are available on both the Slack application and the Slack notifications integration, we don't want new users to set up notifications on Slack notifications * If a user is editing their notifications on Slack notifications, we want to give them as much heads up as possible that this integration will be deprecated ## Proposal For `gitlab.com` users that are currently using `Slack notifications`: - Add a banner to the Slack notifications UI stating that application will soon be deprecated and that they can set up notifications in Slack application. For `gitlab.com` users that have never setup `Slack notifications`: - Remove Slack notifications as an option from the integrations list For `self-managed` users: - Add a banner to the Slack notifications UI stating that application will be deprecated once `Slack application` is available for self-managed. ### Design source - :frame_photo: [Figma project](https://www.figma.com/file/xMkDPODCgaIWllO0GgcdBK/UX%3A-Extend-Slack-app-to-support-notifications-%23335673?node-id=2815%3A28578&t=ajyl7woUYVeBLJvr-1)
epic