chore(v11): add v11 migration guide (#22364)

A first draft version of the v11 migration guide we can collaborate on
and update as we iron out the details.

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: JPeer264 <jan.peer@sentry.io>

Author: 3 people