docs(doc): add v2.1 upgrade guide

[gandalf-at-lerian]

Summary

• Adds the retroactive minor upgrade guide for underwriter from 2.0.0 to 2.1.0-beta.3.
• Patch-only upgrade docs remain ignored, per task scope.

Requested by: @guimoreirar

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

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Run ID: 00207e27-1669-4b15-8fe0-67d1d09cec36

📥 Commits

Reviewing files that changed from the base of the PR and between 3766751 and b0a6b53.

📒 Files selected for processing (1)

• charts/underwriter/docs/UPGRADE-2.1.md

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