The idea is in the link below: https://www.mongodb.com/community/forums/t/mongo-v6-0-0-immediately-exits-with-featurecompatibilityversion-error/181080/4?u=mohammed_khateeb_kamran

Quoting it here:

If you want to save your data, you can fix this, by downloading the compressed archive of 7.0.x and extract it. From this extracted folder you can run ./bin/mongod --dbpath <current database path>. Connect to this instance with mongosh and run db.adminCommand( { setFeatureCompatibilityVersion: "7.0" } ). This will change the FCV for you. You can then exit mongosh and then shutdown the mongod instance and finally start your version 8.0.0 server. You will want to change the FCV here as well to be 7.0.