[Change Proposal] Make transform version number field required

Moving forward, we're altering some of the behavior around transforms in Fleet to better support upgrade paths, backwards compatibility, etc.

As part of these improvements, Fleet will be relying on a version number field for the transform that will determine the behavior during upgrades. This field should be required so if a transform is missing a value its build will fail with a lint error.

cc @qn895 @droberts195 @joshdover @kevinlog