docs: update kernel versions and dates in tables

Every once in a while, we should update the examples
to reflect more recent kernel versions.

Update the tables describing kernel releases, the merge window,
and current longterm maintained kernel, from 2.6-era kernels
to 4.x.

Signed-off-by: Tim Bird <tim.bird@sony.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>