Refactor upgrade/migration instructions in overview.md

[mikadumont]

Removed instructions for starting an upgrade or migration and consolidated stages into a clearer format.

Summary

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Fixes #Issue_Number (if available)

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Internal previews

📄 File	🔗 Preview link
docs/core/porting/github-copilot-app-modernization/overview.md	What is GitHub Copilot app modernization?