Update 90-day blog post

[jsha]

petercooperjr on the forum pointed out that this blog post is quite dated now: https://letsencrypt.org/2015/11/09/why-90-days.html

[petercooperjr]

And it may be that "Why 90 days" should be a documentation thing rather than a blog thing? It's not always clear to me which kind of information to find in a blog post vs. in the documentation.

[tdelmas]

Maybe all blog post older than - for example - one year could had a warning on top with a message like

Warning : this post is X years old, its information may not be accurate or relevant anymore