Clear macOS software update notification?

I’ve recently upgraded a mid-2010 iMac 11,2 to Monterey, and it runs it flawlessly. The only issue I have is there is a badge notification on the system settings icon in the dock, because it wants me to update to Sonoma. I want to sell this machine, and obviously don’t want the purchaser to think they can just upgrade to Sonoma all willy nilly. I’ve tried turning notifications off for the App Store, but that doesn’t clear the little “1” in the dock. Any ideas?