chore: remove board sub-command

[lucarin91]

Motivation

board sub-command is misleading and should be deprecated; all the functionality can be moved to system command.

Change description

arduino@diodio:~$ arduino-app-cli system
Usage:
  arduino-app-cli system [command]

Available Commands:
  cleanup      Removes unused and obsolete application images to free up disk space.
  keyboard     Manage the keyboard layout of the system
  network-mode Manage the network mode of the system
  set-name     Set the custom name of the board
  update       Launches an update of the upgradable packages on the system

Flags:
  -h, --help   help for system

Global Flags:
      --format string      Output format (text, json) (default "text")
      --log-level string   Set the log level (debug, info, warn, error) (default "error")

Additional Notes

Reviewer checklist

• PR addresses a single concern.
• PR title and description are properly filled.
• Changes will be merged in main.
• Changes are covered by tests.
• Logging is meaningful in case of troubleshooting.