ServedPlatform: Exclude<AllPlatforms, "whatnot">

The platforms GET /data/guild/:guildId/:platform can actually answer.

whatnot is in AllPlatforms, and the API route even gates it with canUseFeature('whatnot', ...) before the switch, which makes it look supported from out here. It is not: that switch has arms for kick, twitch, rumble, tiktok, club, twitter and youtube and no whatnot, so a request for it falls through to 400 Invalid platform. Giving whatnot an arm below would describe a response the API cannot produce - this type's own bug, pointed the other way - so it is excluded here instead.

_whatnotStaysUnserved under the type is what fails if someone adds one anyway.