fix: guild member flags can't be set by a bot

[braindigitalis]

Guild member flags cannot be set by a bot. They are set by a user in the client. Sending them causes permission denied. The deaf/muted settings are separate and sent as individual bools in the json, and can be set by a bot.

In fact, the flags that this would set, we don't even have set_ methods for.

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
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