remove `public_only` arg to the contract command in favor of native approach

[dave-connors-3]

          this could be a good opportunity for us to refactor this so that we don't need the `public_only` arg at all. we only use this for the group command, and starting with dbt 1.6+ we can just use `--select access:pubic` instead on this lambda

Originally posted by @dave-connors-3 in #107 (comment)