The v2019 release has reached the end of its support period. It also marks the
point at which we are now fully into the current Apertis release cadence, with
a development, stable and old stable release being supported.

Remove the pointer to the "old old stable" from the main page, the data from
the releases file and the reference to the supported version of Virtual Box.

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