fix: use maxAge instead of expires

[Deckluhm]

According to @fastify/session v9 (the version used by this example), expires is supposed to be a date, not a number.

maxAge can be used instead since it is expecting a number.

Documentation for reference: https://github.com/fastify/session/tree/v9.0.0#cookie

Checklist

• run npm run test and npm run benchmark (doesn't exist)
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct