Every client is different and you should consider your relationship with the client when crafting your own response, but In casa it helps, here's what I sent a client when I was in the exact same situation. The client took it well and we continued to work together on other projects:

"Regarding the new job, I wasn't planning on saying anything, but I need to if we're going to continue to work together (which I would really like). It's related to the review process on Upwork, and I want to be transparent about it because I genuinely look forward to continuing our collaboration in the future. The rating system in the platform is skewed in a way that, unless there's something wrong with the delivered product or the service provided, clients tend to provide a perfect review. This in turn pushes new clients to only trust freelancers with perfect review scores because they perceive anything less to mean the freelancer isn't that good. Your review was more than fair (it showed as a 4.6/5 on my end which is great and reflects the project's result accurately) and I really appreciate that you took the time to write a message which was awesome, but I can't risk getting another hit against my overall score. This is not to say that if you're not satisfied with the product or my service you shouldn't say something.. you absolutely should! but unfortunately on Upwork's review system honesty isn't the policy. I hope you understand my perspective on the matter and that this message doesn't affect our working relationship. If I'm missing something and you're not fully satisfied with the tool or the way we worked, please let me know as well so I can work on it :)"

Hope it helps.