Docs: stop naming a TrueNAS version in the Disk Health note

[SandObserver]

Summary

• Reword the Disk Health TrueNAS note to say later versions lack the REST API, instead of naming version 26. TrueNAS renamed 26.0.0 to 27.0.0.

Testing
npm run build and npm run check pass. Text-only change, no visual check.

Checklist

• npm run build and npm run check both pass.
• Product UI is shown by running or capturing it, never recreated in CSS.
• Any new preview or screenshot carries no private hostnames or real data.
• Visual changes verified in both colour schemes and both docs themes.
• Links to the app repo still resolve.

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