Don’t upgrade yet. The most current version of the macOS is BigSur 11.6.0 and the highest version o Avid MC currently runs only on macOS 11.5.2 which you cannot get right now. I’ve been seeing lots of issues with BigSur 11.6.
That being said… Your MC7 app will not work at all on Catalina or Big Sur. You would need to either buy a new perpetual license, or get a subscription.