CRON_WORKFLOWS_DISABLED env flag for blue/green clusters

# Summary

I am planning a blue-green k8s setup, where I would have 2 k8s clusters at same time, existing workflows on cluster1 should be allowed to finish, and new workflows should only start on cluster2. Trouble I see is both would be starting same cronworkflows. Unless there could be a single controller level flag CRON_WORKFLOWS_DISABLED to allow cluster1 crons to be disabled while cluster2 crons are enabled.

## Use Cases

blue/green k8s clusters. otherwise I need to have 2 repos of cron manifests and suspend many cron yamls on one repo which is a lot more complex. thinking further also want to disable sensors starting new workflows on cluster1 so maybe need a broader NEW_WORKFLOWS_DISABLED flag too

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**Message from the maintainers**:

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No files or tests are named. Start by locating the controller-level handling for CronWorkflows and sensors, then clarify whether CRON_WORKFLOWS_DISABLED or the broader NEW_WORKFLOWS_DISABLED is required; done means preventing new starts on one cluster while allowing existing workflows to finish.

Written by the indexing model from the issue text.