As steevej states, the mongo tool is not included in the download of MongoDB 6.0. You will want to download the mongosh shell now if you wish to interface with the database from the command line. For the most part anything that you could do in the older mongo command line can be done in the newer mongosh tool.
If you installed the default mongodb-org Ubuntu metapackage for MongoDB 5.0+ (per the installation steps above), I believe mongodb-mongosh should be included by default.