Increase Supported Linux Kernel to 5.9 (openzfs#11057)

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

Signed-off-by: Kjeld Schouten-Lebbing <kjeld@schouten-lebbing.nl>

META

@@ -6,5 +6,5 @@ Release:       1
 Release-Tags:  relext
 License:       CDDL
 Author:        OpenZFS on Linux
-Linux-Maximum: 5.8
+Linux-Maximum: 5.9
 Linux-Minimum: 2.6.32