Mark old releases as deprecated and remove dead links

We have release notes for lots of old releases on the website. These are kept
as they show the project has significant history, however a lot of the
information contained in them is likely to be very out of date. Add a warning
to the top of the old releases to note that they are deprecated and remove any
now dead links.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>