Should I disclose my mental illness and medication in a PhD position interview?
There is no requirement to disclose and given the current climate around mental health, I would not suggest disclosing.

I would say that you had health problems that resulted in your having to leave your former school, but that these issues are under control and you are confident you will succeed at your new program.

I would be sure you tell your letter writers to also use the vague rubric of "health problems" as some may inadvertently disclose without thinking about how you want to be framed.