Destroy Staging Workflow when we destroy the main project

When we deleted a project associated to a Staging Workflow, this staging
was not deleted. That caused that we couldn't delete the remaning staging
projects.

Fix #8729.

src/api/app/models/project.rb

@@ -86,7 +86,7 @@ def autocomplete(search)
   has_many :target_of_bs_request_actions, class_name: 'BsRequestAction', foreign_key: 'target_project_id'
   has_many :target_of_bs_requests, through: :target_of_bs_request_actions, source: :bs_request
 
-  has_one :staging, class_name: 'Staging::Workflow', inverse_of: :project
+  has_one :staging, class_name: 'Staging::Workflow', inverse_of: :project, dependent: :destroy
 
   default_scope { where('projects.id not in (?)', Relationship.forbidden_project_ids) }