virtual/linux-sources: Use 6.6 as default kernel

BUG=b/316205830
TEST=presubmit, validation
RELEASE_NOTE=None

Change-Id: Id7f1a163e4538bd353506cf901b0a2fdcdcebaae
Reviewed-on: https://cos-review.googlesource.com/c/cos/overlays/board-overlays/+/76799
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Oleksandr Tymoshenko <ovt@google.com>