linux-gbmc(lts): 6.6 -> 6.12

Upgrade to the next lts version so we keep receiving updated patches.
Nothing is using this version in the default build, so it's trivial to
move forward.

Tested: Booted on AMD Rome, Verified that the sensors and FRU devices
look as expected. Networking is still working and we don't see systemd
service failures.

Google-Bug-Id: 324970585
Change-Id: I67a5e9bbf6c4157aa6b9494737798e4ddc02826d
Signed-off-by: William A. Kennington III <wak@google.com>