feat: Remove the experiment flag for the sandbox commands

[vegeris]

Changelog

This is effectively the release of the sandbox commands and should be highlighted in the changelog:

slack sandbox create
slack sandbox delete
slack sandbox list

Summary

(Please describe the goal of this pull request and mention any related issue numbers)

Testing

Instead of:

lack sandbox list --experiment=sandboxes

We can run:

lack sandbox list

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.14%. Comparing base (809c2ee) to head (b9cf57f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #512      +/-   ##
==========================================
- Coverage   71.18%   71.14%   -0.04%     
==========================================
  Files         222      222              
  Lines       18680    18662      -18     
==========================================
- Hits        13297    13277      -20     
- Misses       4202     4204       +2     
  Partials     1181     1181              

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