Change guild_only to dm_permission in to_dict

[plun1331]

Summary

guild_only is not a valid field in the app commands structure expected by Discord, it expects dm_permission instead.

Fixes #1367

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, typehinting, examples, ...)