doc: update oldest supported kernel in Linux guide

doc: update oldest supported kernel in Linux guide

The DPDK needs to stay up to date with current LTS Linux kernel support.
If the kernel is older than LTS it is likely to be insecure and buggy.
Therefore only require DPDK to work on oldest LTS kernel.

If distribution vendors want to support DPDK on older kernels, that is
their choice. But the upstream source does not need to be cluttered
with support for this.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Marko Kovacevic <marko.kovacevic@intel.com>