Auto Bump Guardian and Garden submodules when a new version of garden-runc-release is available

[aminjam]

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Summary

We have a concourse job that verifies if Garden and Guardian submodules are out of sync with the latest garden-runc-release. In order to update the SHA, it requires a human intervention to bump to the latest. We should be able to automate this, so that when a new version of garden-runc-release is available, we bump the submodules in diego-release.

Expected Result

When the next version of garden-runc-release is out, we no longer have to bump the Garden and Guardian submodules, and validate-garden-version job remains green

[robertjsullivan]

The pipeline job is now working. Garden and Guardian are currently up to date, so we'll have to keep an eye on the job next time https://github.com/cloudfoundry/garden-runc-release posts a new release. Closing out the issue.