Erdős Problem 506: status mismatch (repo=open, erdosproblems.com=solved)

The status of [Erdős problem 506](https://www.erdosproblems.com/506) appears to have changed.

- **[This repo](http://github.com/google-deepmind/formal-conjectures/blob/main/FormalConjectures/ErdosProblems/506.lean)**: `open` (in `FormalConjectures/ErdosProblems/506.lean`)
- **[erdosproblems.com/506](https://www.erdosproblems.com/506)**: `solved`

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

Open FormalConjectures/ErdosProblems/506.lean and inspect its @[category research ...] annotation. Verify the current status of problem 506 on erdosproblems.com, then update the annotation if the external status is confirmed and ensure the file remains valid.

Written by the indexing model from the issue text.