Check for stemcell 3468.21
Check for stemcell 3468.21

This is the current version in use-compiled-releases.yml.

Signed-off-by: Peter Goetz <1bf679f6f155949fea19af278a2667a47961da6b@gmail.com>

cloudfoundry-incubator/bits-service-ci,cloudfoundry-incubator/bits-service-ci,cloudfoundry-incubator/bits-service-ci