META: set Linux minimum version to 4.18

This sets RHEL8's base kernel[1] as the floor, and includes the oldest
kernel.org LTS (4.19).

1. https://access.redhat.com/articles/3078#RHEL8

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16479