Update Crostini release version and channel.

We want to release to dev/canary for version 67 and 68 and to release to
all chanels since version 69.

The beta/stable release is pushed back from 68 to 69.
TEST=manual test.

Change-Id: I02851521a0b9266157be3c4eb510546637b538a1
Reviewed-on: https://chromium-review.googlesource.com/1100539
Reviewed-by: Dan Erat <derat@chromium.org>
Commit-Queue: Tim Zheng <timzheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567321}
1 file changed