feat: point project command users to runpod-flash

[TimPietruskyRunPod]

Summary

The project command is deprecated and hidden since v2.0.0, but it still silently works. Users who try runpodctl project should be told that projects have been replaced by Flash and pointed there instead.

Suggested behavior

When a user runs any runpodctl project subcommand, show a message like:

The "project" command has been removed.

Use Flash for serverless development workflows:
  https://github.com/runpod/flash

And exit without executing the old project logic.

Context

• Related closed issues: Files of projects are not synced #195, Inconsistent Working Directory Behavior Between Development and Production for projects #173
• The project code is currently hidden via Hidden: true in cmd/root.go but the implementation still exists in cmd/project/
• This is also a good opportunity to fully remove the old project source files (cmd/project.go, cmd/project/)