Improvement: add a note about compatibility setting to OSS to Cloud migration guide

[Blargian]

Summary

Closes #4478. Adds a note about compatibility to the OSS to Cloud migration guide.

Checklist

• Delete items not relevant to your PR
• URL changes should add a redirect to the old URL via https://github.com/ClickHouse/clickhouse-docs/blob/main/docusaurus.config.js
• If adding a new integration page, also add an entry to the integrations list here: https://github.com/ClickHouse/clickhouse-docs/blob/main/docs/integrations/index.mdx

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Project	Deployment	Review	Updated (UTC)
clickhouse-docs	Ready	Preview	Jan 7, 2026 0:52am

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