Make previousStageJobsOrNeeds return union

What does this MR do and why?

Updates the return type for previousStageJobsOrNeeds to be a BuildNeedType instead of a JobType. This caused an incident because the previous_stage_jobs_or_needs method was returning Ci::BuildNeed objects instead of the jobs that matches the defined needs. Ci::BuildNeed does not have a type method, so when JobType tried to calculate the id using the below method, a NoMethodError was thrown.

       def id                                                                                                                                          
         return unless object.id.present?                                                                                                              
                                                                                                                                                       
         model_name = object.type || ::CommitStatus.name                                                                                               
         id = object.id                                                                                                                                
         Gitlab::GlobalId.build(model_name: model_name, id: id)                                                                                        
       end 

This MR also updates the spec with an id to prove that this is working now.

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#348287 (closed)