cross-repo-cherrypick: Update downstream branch

ChromeOS has switched to using the main branch, update accordingly.

BUG=b:294218930
TEST=None

Change-Id: I31f67ef4fb175a4e4896b5bed81d5ae1cdddb827
Signed-off-by: Jon Murphy <jpmurphy@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79143
Reviewed-by: Eric Lai <ericllai@google.com>
Reviewed-by: Subrata Banik <subratabanik@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>

util/scripts/cross-repo-cherrypick

@@ -28,7 +28,7 @@ case "${ORIGIN_HOST}" in
 		MESSAGE_PREFIX="UPSTREAM: "
 		;;
 	chromium-review.googlesource.com)
-		BRANCH="cros/chromeos-2016.05"
+		BRANCH="cros/main"
 		MESSAGE_PREFIX=""
 		;;
 esac