Deprecate Rahu

BUR #22745 has been approved by @nonamethanks.

deprecate rahu

Besides a random deleted robot post decades ago, we have two main tags for characters going by "Rahu". Deprecating this just to be safe (and to save people headaches later).

The wiki requirement is there for a reason, just writing "ambiguous tag" is not enough.

Fixed. Sorry about that.

The bulk update request #22745 (forum #266557) has been approved by @nonamethanks.