Keeping the site up to date is a big challenge and I don't think we have a complete plan and policy for doing it. Thanks for raising this topic!
I think most people would agree that we don't want to destroy all out-of-date answers. At the moment, we usually leave them, and add up-to-date answers, or updates to answers to the same question, or create, maintain and link to (and potentially close old and future questions) against a new question. Also, as pointed out by Takkat, many active users are very willing to update and maintain their answers.
You've given useful context, but haven't referenced (a) specific post(s), so here are some general ideas that may be applicable depending on the case. These suggestions don't require reputation, except where indicated.
- Post a new answer to the same question giving the up-to-date solution. If appropriate, suggest an edit to the question to make it more general. (posting an answer requires 10 reputation earned on this site in the relatively rare case that the question is protected, which will be indicated in a banner under the question)
- Suggest an edit to the answer indicating that it works for versions up to [...]. If you have a link to a newer question with up-to-date answers, or a link to an up-to-date answer to the same question, you could also post it in the question. In some cases, for example where the answer already has solutions for multiple versions and the differences are small, it may be appropriate to add a section to the answer with the new solution. All such edits will be subject to review and may not be approved. Make sure you respect the original author, and include as detailed an edit summary as the character limit allows to justify the change.
- Post a new question (as suggested by Sergiy Kolodyazhnyy) linking to old questions and answers and indicating that they do not work for new versions.
- Post on meta asking for the community to do something about the old questions/answers, detailing the problem with links to the specific post(s) in question. Other users may know where to find newer answers, or otherwise have good ideas about what should be done. (posting on meta requires 5 reputation)
- Flag the old question as a duplicate of a question with newer solutions (flagging requires 15 reputation)
- Flag the least useful question for moderator attention and suggest that it be merged into the better one, so that old and new solutions appear together under the same question. (flagging requires 15 reputation) If the request is completed suggest edits to answers where needed to indicate version specificity. Upvote the new answers if they work, so they rise up the view order (upvoting requires 15 reputation).
- Comment on the old question pointing out that answers are outdated and newer solutions can be found
[in answer to this newer question](https://askubuntu.com/questions/SomeNumber/question-about-this-with-new-solutions). (commenting requires 50 reputation)*
*I'd like to point out that we don't expect much value from comments, and while they are often very useful for improving posts and other moderation purposes, they are intended to be temporary. Important information usually belongs in questions and answers. However, comments are sometimes used to link questions, in the form of a link to related posts. This might be appropriate under old questions where a newer question exists, if it doesn't seem appropriate to close the old question as a duplicate of the newer question. Ideally, there should be a more robust way of linking related questions that are not duplicates.