discord-suggestions: use `cleanContent` instead of `content` to resolve mentions

Docs: [`Message#cleanContent`](https://discord.js.org/docs/packages/discord.js/14.14.1/Message:Class#cleanContent), [`Message#content`](https://discord.js.org/docs/packages/discord.js/14.14.1/Message:Class#content)
Code in question: [`modules/etc.ts:45`](https://github.com/typescript-community/community-bot/blob/master/src/modules/etc.ts#L45)

Likely blocked by #91