docs(truenas-csi): mark democratic-csi migration done

[surrealwolf]

democratic-csi and its truenas-nfs StorageClass are gone from every cluster (DataKnifeAI/rancher-deploy#39). Adds a short "Done" note to docs/truenas-csi/migration.md so nobody follows the old migration steps; the StorageClass/postgres notes remain valid. Docs only.

Made with Cursor

[cursor]

Approved. Cursor Bugbot completed successfully with no findings that need human review, and no approval policy requires additional review. Docs-only change on docs/truenas-csi/migration.md.

  

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