Documentation: Fix kernel support matrix

Documentation: Fix kernel support matrix

The documentation matrix for OVS branches and which kernels they support
is out of date.  Update it to show that since 2.10 the lowest kernel
that we test and support is Linux 3.16.

RHEL and CentOS kernels based upon the original 3.10 kernel are still
supported.

Reported-by: Han Zhou <hzhou@ovn.org>
Reported-at: https://mail.openvswitch.org/pipermail/ovs-dev/2020-May/370742.html
Acked-by: Han Zhou <hzhou@ovn.org>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: William Tu <u9012063@gmail.com>