Add user-facing message when deprecated cban alias is used

[neevets]

This adds a helpful response when users attempt to use the deprecated cban
alias, explaining why it was removed and suggesting the correct alternatives
(cleanban or compban).

This should reduce confusion, especially for long-time moderators.

[jb3]

I'm tempted to suggest we just add cban back as an alias for cleanban.

Quite a few moderators are slipping up on this still, if that's the case, the solution is probably to revert to actually having the alias than some deprecation message (moderators can figure out it's deprecated based on the bot not banning a user).

[neevets]

Fixes #3458

[jb3]

After speaking with Moderators, we want to bring back the cban alias for the cleanban command.

Please can you adjust this PR to implement this (should just be undoing the change & adding the alias to the list in the cleanban command).

[neevets]

for sure!