If most or all answers to a question are out of date, and you don't want to comment on the answers, you can comment on the question. The authors of the answers won't be notified, but people who come upon the question will be able to consider it, and may contribute to bring it up to date.
If the answers can be fixed in such a way as to conform to the editing guidelines--keep within the scope of the answer and respect the author and the author's intentions--then you can edit them yourself to improve them! Instant win.
Keep in mind that the most common way for answers to be out of date is that they provide information that applies to some Ubuntu releases but not to more recent releases. That doesn't appear to be what's going on in the example you gave...but when that does happen, you can edit the answers to indicate the scope of their applicability. The best way to do this is usually to put a version number, list, or range at the top--that way, people reading through can immediately see which answers apply to which releases. If you can post an answer that applies to whatever releases aren't covered, that's a big plus.
Often it's a greater undertaking to bring answers up to date, or to create an answer that is applicable to more recent releases (or a more recent situation). When this happens, consider the following resources:
- Chat. You can discuss it with others and see if others are able and willing to help. This can lead to people editing or posting answers, or simply to ideas being hashed out or suggestions made that would enable you to add an answer or improve the existing ones.
- Bounties. You can put a bounty on a question to publicize the need for more current answers. The bounty can go to an existing answer once it's edited, or to a new answer. Rather than shouldering the burden of setting all the bounties yourself, you can discuss the need in chat and see if others are willing to put up some of them.
- When the need for revision is pervasive, such that a similar need affects many questions (or one or more of the most important questions on the site), you can post here on meta about the need for revisions.