Call-to-acion for users of "Slack slash commands" integration to move to Slack app

As we complete our implementation of the Granular Bot Permissions Slack App, and add support for both GitLab.com and self-managed users, we will need to deprecate use of our legacy Slack Slash Commands integration, consolidating usage into our new app.

At time of writing, the Slack Slash Commands integration has one benefit over the GitLab for Slack app integration which is that it can be configured at group and instance level. We have Enable GitLab for Slack app for Instances and G... (#391526 - closed) to address this. Although deprecating the integration before we have completed that issue isn't required, we should at least address it shortly after.

Proposal

• frontend backend Add deprecation notices to encourage users of the Slack Slash Commands integration to move across to Slack App integration:
  1. Integrations index (only if Slack Slash Command integration is enabled).
  2. Slack Slash Commands integration edit page.
• backend Prevent any new Slack Notifications integrations from being created, but still allow existing Slack Slash Commands integrations to be edited:
  1. Hide the Slack Slash Commands 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 Slash Commands integration documentation to mention it is deprecated.