ci: switch "stable" build test to compile against kernel 6.8

Following the rationale in #57, switch the stable build test to compile
against 6.8, which is the kernel version that comes in the latest Ubuntu
LTS (24.04).

Related: #57 (comment)
Signed-off-by: Jonas Malaco <jonas@protocubo.io>

Author: jonasmalacofilho

.github/workflows/build.yml

@@ -9,7 +9,7 @@ jobs:
       matrix:
         kernel_version:
           - master
-          - v6.2
+          - v6.8
         kconfig_pm:
           - "-e CONFIG_PM"
           - "-d CONFIG_PM"