If I upgrade to Navicat v12, will my connection and other information automatically sync to Navicat Cloud?

No, nothing will be synced to Navicat Cloud after the v12 update. You will need to create your connections and files according to your preference and sync to the cloud. A Navicat login is also required for access to Navicat Cloud, that being said, you do not need to login/create a Navicat Cloud account if you do not wish to use the Cloud function.