fix(migrations): preserve legacy deployer intent after undeploy

[Elvand-Lie]

Fixes #4054.

Legacy func.yaml files stored the selected deployer only under deploy.deployer. func delete clears that deployment state, so the next deploy falls back to the configured default.

Recover missing top-level deployer intent when migrating files whose specVersion is below 0.39.0. This preserves the selected deployer across delete and redeploy while preserving explicit intent and observed state.

Testing: regression tests cover legacy YAML loading and CLI delete/redeploy for raw, KEDA and Knative, including global defaults and flag/environment overrides. The recovery cases fail on main and pass with this fix; the control cases pass on both. Full Linux make test and make check passed. This PR also makes an existing fingerprint test explicitly advance the file timestamp so rapid writes cannot cause a spurious failure.

Files whose specVersion is already 0.39.0 or later with only observed deployer state are outside this migration's scope. Windows/macOS and cluster tests were not run locally.

/kind bug

Legacy functions retain their selected deployer after delete and redeploy instead of falling back to the configured default.

N/A

[knative-prow]

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