A better fix for subcommands having MISSING cogs

[Middledot]

Summary

A better fix for subcommands not having the cog attribute set.
It's better because SlashCommandGroup._set_cog will set the subcommands cogs as well as its own.

Information

• 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, ...).

Checklist

• I have searched the open pull requests for duplicates.
• 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.

[Dorukyum]

Hope this will be the final solution 😅

[Lulalaby]

I would say GLHF @Dorukyum