[go_router] Add TODOs for meta migration (#9535)
Prepares for: https://github.com/flutter/flutter/issues/171410
I created a calendar event for January 30, 2026. Hopefully I'll still employed then 🤞
**Version change exemption reason**, **CHANGELOG exemption reason**, **Test exemption reason**: This only adds comments. No semantic changes.
## Pre-Review Checklist
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