Flip StorageVersionMigrator default to on before v1alpha1 removal

Follow-up to #4969.

The StorageVersionMigrator controller, opt-in CRD labels + CI guard, chart feature flag, and docs all shipped (PRs #5362, #5391, #5418, #5451). The feature is currently opt-in / default-off:

• cmd/thv-operator/app/app.go — isStorageVersionMigratorEnabled() defaults to false (admins set TOOLHIVE_ENABLE_STORAGE_VERSION_MIGRATOR=true)
• deploy/charts/operator/values.yaml — operator.features.storageVersionMigrator: false

Remaining work (the originally-planned PR-C flip)

Before the release that drops v1alpha1 from the ToolHive CRDs' spec.versions, flip the default to on so every cluster's status.storedVersions is cleaned during the deprecation window:

• Flip operator-code default to true
• Set operator.features.storageVersionMigrator: true in values.yaml (regenerate chart README)
• Verify upgrade-guide docs reflect default-on behaviour and the opt-out path for admins running kube-storage-version-migrator externally

This is gated on release timing, not implementation.

🤖 Generated with Claude Code

Start in cmd/thv-operator/app/app.go and deploy/charts/operator/values.yaml, then locate the chart README and upgrade-guide documentation mentioned in the issue. Confirm the release timing and existing opt-out wording before updating the defaults and regenerated documentation; done means all three checklist items are consistent.