Slack Application integration is only available on GitLab.com and in the
development environment.
https://docs.gitlab.com/ee/user/project/integrations/gitlab_slack_application.html

After adding group-level integrations
&2543, we have to change
this integration to only appear at project level. This is because the
implementation is coupled to projects and in terms of functionally it
also makes sense since:
- At the instance-level on GitLab.com we are not going to configure it.
- At the group-level we don't have to configure it either and propagate
  the values, it is already a one-click installation and we don't have
  to propagate any values to the projects.