Update Convex example to convex@0.12.0 (#47175)
Updates Convex to 0.12.0
### Improving Documentation or adding/fixing Examples
- [x] The "examples guidelines" are followed from our contributing doc https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md
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Co-authored-by: Alex Cole <2695197+alexcole@users.noreply.github.com>
