Increase Supported Linux Kernel to 5.9

This increases the Linux kernel version to 5.9 from 5.8
as most compatibility fixes should already be included.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Pavel Snajdr <snajpa@snajpa.net>
Signed-off-by: Kjeld Schouten-Lebbing <kjeld@schouten-lebbing.nl>
Closes #11050