libkrun v1.19.4 fork adding PVH boot primitives (start_info + protected-mode vCPU state) for faster, kernel-direct x86_64 microVM startup.

ci: switch to ubuntu-26.04 runners master

Let's switch to newer kernels, hoping to avoid the buggy kernel in 24.04 that leads to https://github.com/libkrun/libkrun/issues/314 Signed-off-by: Sergio Lopez <slp@redhat.com>