[Improvement] Warning on old phpBB2 articles

As per https://www.phpbb.com/community/viewtopic.php?f=72&t=2164800

The user didn't know that the article only related to phpBB2 and I must admit that I wasn't totally clear from just scanning over the article, until I reached the db changes required. With one small link in the breadcrumbs to "phpBB2" I don't think it is clear enough that these articles are out of date and not suitable for phpBB3.

I propose that:

a) all articles in the phpBB2 are removed/hidden from public view, support was discontinued nearly 4 years ago.

b) a notice is placed in the header of the articles, similar to the one on this bug tracker stating that the article only applies to phpBB2, with a link to the more recent articles for phpBB3.

One of these changes would improve the user experience and mean that it's less likely we have people trying to follow instructions for an out of date KB article.