fix: Remove deprecation warning on Message#cleanContent

[Jiralite]

Please describe the changes this PR makes and why it should be merged:
Resolves #7142.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

[Seblor]

I'm not familiar with the inner workings of DJS, but from an outsider's point of view, this looks like a stopgap solution.

The Util.removeMentions method is being deprecated because of the introduction of allowedMentions (#6530). So maybe the code from Util.removeMentions should be moved to the Message.cleanContent getter, or the Util.removeMentions should not be deprecated.

[Jiralite]

Message#cleanContent calls Util.cleanContent() which calls Util.removeMentions() which sanitises input to prevent mentionability by a simple str.replaceAll('@', '@\u200b'). Once Util.removeMentions() is removed, this replacement will no longer exist. Message#cleanContent will still call Util.cleanContent() and will still act as it always has - the only difference being that mentions are not sanitised.

The reason for this is in case people send clean content as a new message which may cause @everyone or @here injections. Since allowedMentions exist, that should be the preferred method to prevent mentions in your message.

[Seblor]

That makes sense, thank you for the clarification.

[ImRodry]

Shouldn't cleanContent still return @Rodry#4020 instead of <@240875059953139714>? Maybe it shouldn't only remove mentions but also make them readable outside of Discord

[vladfrangu]

The only thing this PR does is prevent the deprecation warning from showing in .cleanContent, not remove the replace to username behavior

[ImRodry]

Once Util.removeMentions() is removed, this replacement will no longer exist. Message#cleanContent will still call Util.cleanContent() and will still act as it always has - the only difference being that mentions are not sanitised.

I was commenting on this

[vladfrangu]

It means that if you cleanContent a member mention, whose display name is everyone, you will get an @everyone in your chat instead of @\u200beveryone.