Deprecate legacy Slack Notifications integration

Description

This issue is part of the epic &8670 (closed). See that epic for more context.

After #372410 (closed) the Slack Application integration will have the notifications capability of the separate Slack Notifications integration.

This issue is to deprecate the old Slack Notifications integration (SaaS-only users until &1211 (closed)).

Proposal

• frontend backend Add deprecation notices to encourage users of the Slack Notifications integration to move across to Slack App integration:
  1. Integrations index (only if Slack Notifications integration is enabled).
  2. Slack Notifications integration edit page.
• backend Prevent any new Slack Notifications integrations from being created, but still allow existing Slack Notifications integrations to be edited:
  1. Hide the Slack Notifications integration from the list of integrations to choose from, unless they have one created already.
  2. Return an error status code in the API if the PUT /projects/:id/integrations/slack call were to create the integration. (We would allow calls to the endpoint that update existing records).

2. Update the Slack Notifications integration documentation to mention it is deprecated.

Design source

🖼 Figma