SK-2837: Add V2.1 migration guide and README upgrade banner

[Devesh-Skyflow]

Adds docs/migrate_to_v2.md covering auth options, client init, request/response structure, error format, and camelCase field name changes for V1 → V2 migration. Adds README banner pointing V1 users to the guide and noting EOL on 2026-10-31.

Start with a concise summary of the PR. The first three sections are required. The questions present in each section is there to help you guide you what to add. They are meant to be overwritten by your comments.

Why

• Why are you making the change?
• What is the underlying issue that you are trying to case, in case of fix?
• Why is it needed by the feature you are working on?
• What is the intent behind making the change?

Goal

• What is the intended outcome?
• What part of the feature should start working?
• What are the non-goals or will be covered in future PR?

Testing

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• If you haven't written unit tests, why?
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Tech debt

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