Prevent any new Slack Notifications integrations from being created on self-managed

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Close this issue

About

In #392407 we prevented new Slack Notification integrations from being created on Gitlab.com. This issue is to widen this to self-managed also.

Note we do not want to delete customer PostgreSQL data for the Slack Notifications integration as we want to allow them to migrate to the GitLab for Slack app #372412.

This issue is blocked by:

• Self-managed need to be able to use the GitLab for Slack app &1211 (closed).
• People need to be able to configure the GitLab for Slack app at group and instance-level #391526 (closed).
• We need to allow people to easily migrate their existing settings from the Slack notifications integration to the GitLab for Slack app integration #372412.