Account for $wgSquidMaxage in OutputPage::checkLastModified().

* This deals with the fact that seldom edited pages can end up cached
  with very stale resource (JS/CSS) references since the response to
  IMS GET requests will be 304 Not Modified if page_touched is ancient.
  When squid revalidates its stale cache it will keep getting 304s and
  renewing the TTL on the stale cache.

Bug: 44570
Change-Id: I3889f300012aeabd37e228653279ad19b296e4ae