Here's my two cents on this.  I'm going to talk in very general terms, because the question applies not just to one you linked, but to many other out there. And remember, this is just my personal opinion on this. 

Voting is the measure of *usefulness* of an answer, not of whether an answer is outdated. If you ask a current question and receive outdated answer, then logically it is not useful - down-voting that is OK. 

Downvoting an outdated answer on outdated question, however, is not logical and achieves no useful purpose (especially if downvoted without comment). At the time of writing it may have been both useful and relevant. Best course of actions in my opinion (*disclamer, this is just an opinion*) would be: 

- leave a comment requesting the author to update the answer; especially in case of high-reputation users, there's a high probability they're active on the site and will see your comment
- if the user haven't answered and you don't have editing privileges - bring it up to the attention of users in [Ask Ubuntu General Chatroom][1] or [Downboat Chatroom][2]. Alternatively, request it here on meta site, especially if you are not sure how to proceed or how exactly information should be edited. After all, Ask Ubuntu is a community, so talk to people.
- if you do have the privilege to edit posts - do that, however do so in non-destructive way; that means, **do not delete the original content of the answer**. Remember, the answer in question was relevant in the past, so it may have (1) historic value, (2) it may be referenced by other more recent posts discussing what has been going in the past, (3) believe it or not, there are people out there who still use legacy Ubuntu releases or legacy software, and the solution may actually be useful to them. 

**NOTE**: Please do not flag posts for moderator attention; this is not their job to keep answers up to date (unless the answer could be potentially harmful, then it could be argued that it's under their jurisdiction). In other words, don't bother moderators with what community or high rep users can do alone.

***

Now, what about downvoting itself ? You certainly *can* do that, and you can get away with the excuse  "I'm downvoting the answer because it is not useful *anymore*". That's all fair and square, voting is anonymous, and you don't have to even explain why you downvoted an answer. 

However downvoting without explaining achieves no useful effect, and you really need to figure out what is your purpose - do you just want to silently disagree, or actively partake in making information relevant. You downvote an outdated answer, OP might see it and just ignore it; old answer, who cares. By contrast, if you downvote and leave a comment, if the author is on the site they will update the answer. But then again, downvoting isn't even necessary. 

In other words, when it's about relevance/up-to-date-ness of an answer, voting alone isn't useful when done by itself. That's why best course of action is editing and communicating with author of the post.

  [1]: http://chat.stackexchange.com/rooms/201
  [2]: http://chat.stackexchange.com/rooms/3877