Linux 6.8 compat: META (#16099)

Update the META file to reflect compatibility with the 6.8 kernel.

Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>

META

@@ -6,5 +6,5 @@ Release:       1
 Release-Tags:  relext
 License:       CDDL
 Author:        OpenZFS
-Linux-Maximum: 6.7
+Linux-Maximum: 6.8
 Linux-Minimum: 3.10