Update SDK to 18603.0.0

BUG=b/354251827
TEST=presubmit
RELEASE_NOTE=None

Change-Id: I8fe5ff10c0a2ceba00e7f32a646c4e1414dc5c9c
Reviewed-on: https://cos-review.googlesource.com/c/cos/overlays/board-overlays/+/78419
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>