Erdős Problem 6: status mismatch (repo=solved, erdosproblems.com=formally solved)

The status of [Erdős problem 6](https://www.erdosproblems.com/6) appears to have changed.

- **[This repo](http://github.com/google-deepmind/formal-conjectures/blob/main/FormalConjectures/ErdosProblems/6.lean)**: `solved` (in `FormalConjectures/ErdosProblems/6.lean`)
- **[erdosproblems.com/6](https://www.erdosproblems.com/6)**: `formally solved`

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

Open FormalConjectures/ErdosProblems/6.lean and compare its current status annotation with the status shown at erdosproblems.com/6. Verify whether the problem is formally solved, then update the @[category research ...] annotation if the external status is confirmed.

Written by the indexing model from the issue text.