use GroupCog instead of cog and group

[TobiasGoetz]

Is your feature request related to a problem? Please describe. See https://discordpy.readthedocs.io/en/stable/ext/commands/api.html?highlight=cog#groupcog

Describe the solution you'd like A clear and concise description of what you want to happen.

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

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[TobiasGoetz]

Already implemented