Add created_by relationship to User model

What does this MR do and why?

Related to: https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/772

This MR refactors the user model by implementing a belongs_to relationship for created_by and removing the created_by method. This was found during an MR review which required mocking created_by in tests to allow them to pass. By implementing this we are also able to remove the mocks that were used in the specs.

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