Change StudentProgramAssociation references to reference base class

In 2.0, there are two choice types (StudentLearningObjectiveSectionOrProgramChoice and StudentCompetencyObjectiveSectionOrProgramChoice) that reference StudentProgramAssociation. In 3.0, an abstract base class was created called GeneralStudentProgramAssociation and StudentProgramAssociation is now a subclass of that base class, used to capture generic program enrollments. These references now need to be updated to refer to the base class (GeneralStudentProgramAssociation) rather than StudentProgramAssociation to maintain the model intent for 2.0.