feat: Rename goup default to goup set

[ForrestSu]

the end.

Available Commands:
  completion  Generate the autocompletion script for the specified shell
  help        Help about any command
  install     Install Go with a version
  ls          List all installed Go
  remove      Remove Go with a version
  search      Search Go versions to install
  set         Set the default Go version
  upgrade     Upgrade goup
  version     Show goup version

Flags:
  -h, --help      help for goup
  -v, --verbose   Verbose