CI: Upgrade kernel to v5.14

Older kernels cannot be built and booted with GCC 10+, as included in
Debian bullseye, due to unfortunate stack-canary issues:
    https://lore.kernel.org/lkml/20200314164451.346497-1-slyfox@gentoo.org/T/



Upgrade to the last-released kernel, 5.14, to fix this.

Signed-off-by: Daniel Stone <daniels@collabora.com>