Checkmk Werk 15676: group_config: delete endpoints now return a 409 when is use.

ID: 15676 Title: group_config: delete endpoints now return a 409 when is use. Component: Core & setup Level: 1 Class: Bug fix Version: 2.3.0b1 This werk introduces a fix to the contact_group_config, host_group_config and service_group_config delete endpoints. When the groups were in use, they were incorrectly returning a 400 response. We now return a 409 Conflict response.