UBUNTU: [Config] disable CONFIG_KFENCE_STATIC_KEYS

UBUNTU: [Config] disable CONFIG_KFENCE_STATIC_KEYS

BugLink: https://bugs.launchpad.net/bugs/1948038
KFENCE devs are changing CONFIG_KFENCE_STATIC_KEYS to be disabled by
default, because it's introducing more problems than benefits:

https://lore.kernel.org/all/20211019102524.2807208-2-elver@google.com/T/#u

This also seems to trigger QEMU bugs during the systemd autopkgtest, like:
https://bugs.launchpad.net/qemu/+bug/1920934

Even if this may introduce a small overhead in terms of performance it
seems safer to keep KFENCE_STATIC_KEYS disabled for now, and it is still
a better solution than disabling KFENCE completely.

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>