Tests: Investigate `OrderedModelManager` warning for Sponsor models

[JacobCoffee]

Investigate separately these non-failing warnings

WARNINGS:
SponsorshipBenefit: (ordered_model.W003) OrderedModelBase subclass has a ModelManager that does not inherit from OrderedModelManager. This is not ideal but will work.
SponsorshipPackage: (ordered_model.W003) OrderedModelBase subclass has a ModelManager that does not inherit from OrderedModelManager. This is not ideal but will work.

Originally posted by @JacobCoffee in #2741 (comment)

[Irfan-Ansari-stm]

Hi @JacobCoffee, I’d be happy to tackle this cleanup. I can update both SponsorshipBenefit and SponsorshipPackage to use managers that inherit from OrderedModelManager. Would you mind assigning this issue to me? Let me know if there’s anything else I should be aware of before I open a PR. Thanks!