I forgot to set the featurecompatibilityversion while upgrading Mongodb from 7 to 8. Now I can’t use mongosh. How to solve this?

While upgrading, I forgot to set the featurecompatibilityversion while upgrading Mongodb from 7 to 8. Now I can’t use mongosh. How to solve this? The installation is done in Ubuntu and the data is important so I need to save it. I tried using mongod --repair but it didn’t help.