Update Convex Example

[ianmacartney]

Updates the convex example to use a more recent version, along with newer style syntax and best practices.

Adding or Updating Examples

• The "examples guidelines" are followed from our contributing doc https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md
• Make sure the linting passes by running pnpm build && pnpm lint. See https://github.com/vercel/next.js/blob/canary/contributing/repository/linting.md

[ianmacartney]

I can't get info from those errors: is it just b/c this is on my personal org, not owned by next.js?

[leerob]

Looks like Prettier failed here 🙏

[ianmacartney]

Very strange. That failing file was only touched by the merge commit. Rebasing w/o that merge commit and running pnpm lint passes without changing anything. ¯_(ツ)_/¯