Erdős Problem 571: status mismatch (repo=open, erdosproblems.com=formally solved)

The status of [Erdős problem 571](https://www.erdosproblems.com/571) appears to have changed.

- **[This repo](http://github.com/google-deepmind/formal-conjectures/blob/main/FormalConjectures/ErdosProblems/571.lean)**: `open` (in `FormalConjectures/ErdosProblems/571.lean`)
- **[erdosproblems.com/571](https://www.erdosproblems.com/571)**: `formally solved`

Please verify and update the `@[category research ...]` annotation if appropriate.

Check the current status of Erdős problem 571 at erdosproblems.com/571, then inspect the @[category research ...] annotation in FormalConjectures/ErdosProblems/571.lean. Update the annotation if the external status is confirmed and verify that the repository no longer reports the outdated status.

Written by the indexing model from the issue text.