zephyr: guybrush: Drop support for Zephyr v2.6
zephyr: guybrush: Drop support for Zephyr v2.6

This allows us to use the v2.7 pinmux APIs without breaking the v2.6
build.

BUG=b:195137794
BRANCH=none
TEST=build for guybrush

Signed-off-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Change-Id: Ib35fe7ccebc0445fc0167b47d6533dd7981fcb0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3194524
Reviewed-by: Diana Z <cb2016afc7183e5356c690dc63b4ba817470501a@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec